Comment 80 for bug 291040

this is written in some unspecified docs, however 2 has never made sense to me.

1. After BITBLT_MULTI copy data from frame buffer to system memory, flush entire 2D pixel cache and
   wait for 2D engine idle and clean to ensure the copied data to arrive into bus controller.
2. Copy the last pixel back to frame buffer from system memory to flush data out of bus controller to system
   physical memory.
3. Wait for flush process to be completed. This is done by either waiting for engine idle or using timestamp
   write back mechanism.