i965/blorp: Define a function to clamp texture coordinates

i965/blorp: Define a function to clamp texture coordinates

New function clamp_tex_coords() clamps the texture coordinates
to texture boundaries.  This function will also be utilized later
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>