radeon/amdgpu pageflip regression fixes for Linux 4.4+

[Mario Kleiner]

Two more patches for radeon-kms/amdgpu-kms to fix some new
regression introduced into Linux 4.4 by other fixes for
the vblank regressions in 4.4.

These fix fdo bug 93746 filed against Linux 4.4.

thanks,
-mario