R600/SI: Use unnormalized coordinates for sampling with the RECT target.

Patch by: Michel Dänzer

Reviewed-by: Tom Stellard <thomas.stellard@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Michel Dänzer <michel.daenzer@amd.com>
llvm-svn: 173053