radeonsi: avoid redundant CB and DB register updates

radeonsi: avoid redundant CB and DB register updates

The main idea is to avoid setting CB_COLORi_INFO = 0 for i>0 repeatedly
when those colorbuffers aren't used. This is mainly for glamor.

Same for DB. Z_INFO and STENCIL_INFO need to be cleared only once.

Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Acked-by: Christian König <christian.koenig@amd.com>