Hi,
I have eventually performed the commit bisect (sorry it took me so long time) between 3.19.0-16 and 3.19.0-17 as the last was the one where I first had the problem.

here is git message after last git bisect command :
-----------------
5cfb2705fc0c05b182f523b2aee5143c4b402536 is the first bad commit
commit 5cfb2705fc0c05b182f523b2aee5143c4b402536
Author: Matthew Garrett <email address hidden>
Date:   Fri Apr 24 14:59:01 2015 +0800

    PCI: Don't clear ASPM bits when the FADT declares it's unsupported
    
    Communications with a hardware vendor confirm that the expected behaviour
    on systems that set the FADT ASPM disable bit but which still grant full
    PCIe control is for the OS to leave any BIOS configuration intact and
    refuse to touch the ASPM bits.  This mimics the behaviour of Windows.
    
    BugLink: http://bugs.launchpad.net/bugs/1441335
    
    Signed-off-by: Matthew Garrett <email address hidden>
    Signed-off-by: Bjorn Helgaas <email address hidden>
    (cherry picked from commit 387d37577fdd05e9472c20885464c2a53b3c945f)
    Signed-off-by: Alex Hung <email address hidden>
    Acked-by: Colin King <email address hidden>
    Acked-by: Tim Gardner <email address hidden>
    (backported from commit 387d37577fdd05e9472c20885464c2a53b3c945f upstream)
    Signed-off-by: Brad Figg <email address hidden>

:040000 040000 9c1764a2e96361d082806c42b03e848facf4914e a77df148e590328d4a76f775f95a6353bfebb563 M	drivers
:040000 040000 069a2e3a077ee8342264ef9da0df5e3d97f94eaf e22c053cb941d5409e27a39590bf8413f2bb2fac M	include
--------------------------------------------------------

and here is the output of git bisect log :

-------------------------------------------------------
git bisect log
# bad: [0a19ec4f3046a2c5955578ba54cc3067876fe408] UBUNTU: Ubuntu-3.19.0-17.17
# good: [ab7ec15d305950bf911023d72eb7d225593bf688] UBUNTU: Ubuntu-3.19.0-16.16
git bisect start 'Ubuntu-3.19.0-17.17' 'Ubuntu-3.19.0-16.16'
# good: [205c4ddd68190c2433a7636224f6882fa312fd83] radeon: Do not directly dereference pointers to BIOS area.
git bisect good 205c4ddd68190c2433a7636224f6882fa312fd83
# bad: [d359cc6fa8c33b397ae4dc881aad494d185d58ee] drm/atomic-helper: add connector->dpms() implementation
git bisect bad d359cc6fa8c33b397ae4dc881aad494d185d58ee
# good: [77194b8d152b4e8e32bffe81b6f7574ce78f9cde] drivers/of: Add empty ranges quirk for PA-Semi
git bisect good 77194b8d152b4e8e32bffe81b6f7574ce78f9cde
# good: [057521712bbe7b7d05fc27753242edefbf704063] clk: divider: fix calculation of maximal parent rate for a given divider
git bisect good 057521712bbe7b7d05fc27753242edefbf704063
# good: [b5e9721ffc043b02d9dbf5cec47ad38803a3a4f6] UBUNTU: SAUCE: vesafb: Set mtrr:3 (write-combining) as default
git bisect good b5e9721ffc043b02d9dbf5cec47ad38803a3a4f6
# bad: [4547f0a3499ac87aae348bd06fb48abfecd81287] i915_bpo: Update to 4.1rc1
git bisect bad 4547f0a3499ac87aae348bd06fb48abfecd81287
# bad: [ae516dc61ed82132df1bca378a9b971770b4a242] regmap: Skip read-only registers in regcache_sync()
git bisect bad ae516dc61ed82132df1bca378a9b971770b4a242
# bad: [5cfb2705fc0c05b182f523b2aee5143c4b402536] PCI: Don't clear ASPM bits when the FADT declares it's unsupported
git bisect bad 5cfb2705fc0c05b182f523b2aee5143c4b402536
# first bad commit: [5cfb2705fc0c05b182f523b2aee5143c4b402536] PCI: Don't clear ASPM bits when the FADT declares it's unsupported
---------------------------------------------------------------------------------

I turned status back to confirmed