I see the CONFIG_DRM_RADEON_UMS kernel option was disabled in Utopic but not Trusty. However, you did not see the bug until after 3.16.0-17, but CONFIG_DRM_RADEON_UMS was also not set in 3.16.0-17.
I think our best bet is to bisect between v3.16.3 and v3.16.4. It will require testing about 8 - 10 kernels. I'll build the first test kernel and post a link to it shortly.
I see the CONFIG_ DRM_RADEON_ UMS kernel option was disabled in Utopic but not Trusty. However, you did not see the bug until after 3.16.0-17, but CONFIG_ DRM_RADEON_ UMS was also not set in 3.16.0-17.
I think our best bet is to bisect between v3.16.3 and v3.16.4. It will require testing about 8 - 10 kernels. I'll build the first test kernel and post a link to it shortly.