Pageflip fixes for the amd dal/dc staging tree.

[Mario Kleiner]

Hi

These two patches were written and tested against Harry's
hwentland amd-staging-dc-drm-next branch from a few days ago.
Not sure if that is the right one to base dal/dc patches against,
but it looks recent and active enough?

They fix pageflip scheduling to always flip at the right target vblank
and pageflip completion events to send out the correct vblank count
and timestamp of flip completion.

Successfully tested on AMD R9 380 Tonga Pro - VI/DCE-10. After applying
these, the dal staging branch performs nicely with my tests :)

I've also tested amdgpu-pro 17.10.2 on the same setup against Linux 4.4/4.8
and found a couple of similar bugs and fixes, but that in a separate mail.

-mario