drm/amd/include:cleanup raven1 gc header files.

Cleanup asic_reg/raven1/GC folder. Remove unused files:
    gc_9_1_default.h  gc_9_1_sh_mask.h

Signed-off-by: Feifei Xu <Feifei.Xu@amd.com>
Acked-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Junwei Zhang <Jerry.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>