i965/blorp: Use more appropriate variable names

i965/blorp: Use more appropriate variable names

When we talk about both multi-sample and single-sample scaled blits,
rect_grid_{x1, y1} are more appropriate variable names as compared
to sample_grid_{x1, y1}. There are no functional changes in this patch.
It just prepares for the BLORP implementation of single-sample scaled
blit with bilinear filter.

Signed-off-by: Anuj Phogat <anuj.phogat@gmail.com>
Reviewed-by: Paul Berry <stereotype441@gmail.com>