radeonsi: Maintain cache of pixel shader variants according to contxt state.

radeonsi: Maintain cache of pixel shader variants according to contxt state.

Mostly inspired by r600g commit 4acf71f01ea1edb253cd38cc059d4af1a2a40bf4
('r600g: cache shader variants instead of rebuilding v3').

Signed-off-by: Michel Dänzer <michel.daenzer@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>