gallium: Fix VAAPI postproc blit
Fixes the VAAPI postproc issue mentioned in this comment (!6736 (merged)) without changing the height of the underlying resource when doing the blit.
This commit removes the 0.5 pixel center offset from the compute blit - VAAPI postproc is the only function that uses this compute blit.
Signed-off-by: Thong Thai thong.thai@amd.com