radeonsi: remove interp_at_sample from the key, use TGSI_INTERPOLATE_LOC_SAMPLE

radeonsi: remove interp_at_sample from the key, use TGSI_INTERPOLATE_LOC_SAMPLE

st/mesa has the same flag in its shader key, we don't need to do it
in the driver anymore.

Instead, use TGSI_INTERPOLATE_LOC_SAMPLE, which is what st/mesa sets.

Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>