gallium: Fix VAAPI postproc blit

gallium: Fix VAAPI postproc blit

Fixes the VAAPI postproc issue mentioned in this comment
(https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6736#note_626808)
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.

Fixes: 49465babdb3 ("frontends/va/postproc: Use the actual image height when blitting")
Signed-off-by: Thong Thai <thong.thai@amd.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/7970>