radeonsi: fix out-of-bounds indexing of shader images

radeonsi: fix out-of-bounds indexing of shader images

Results are undefined but may not crash. Without this change, out-of-bounds
indexing can lead to VM faults and GPU hangs.

Constant buffers, samplers, and possibly others will eventually need similar
treatment to support GL_ARB_robust_buffer_access_behavior.

Reviewed-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Reviewed-and-Tested-by: Michel Dänzer <michel.daenzer@amd.com>