i965/blorp: Add support for single sample scaled blit with bilinear filter

Currently single sample scaled blits with GL_LINEAR filter falls
back to meta path. Patch removes this limitation in BLORP engine
and implements single sample scaled blit with bilinear filter.
No piglit, gles3 regressions are obeserved with this patch. Piglit
test case patches to verify this implementation are out on piglit
mailing list.

V2: Use "sample" message to utilize the linear filtering functionality
built in to hardware.
V3: Define a bool variable (bilinear_filter) to handle the conditions
for GL_LINEAR blits.

Signed-off-by: Anuj Phogat <anuj.phogat@gmail.com>