drm/amdgpu: Modify GC register access from MMIO to RLCG in file soc15.c

In SRIOV environment, KMD should access GC registers
with RLCG if GC indirect access flag enabled.

Signed-off-by: Peng Ju Zhou <PengJu.Zhou@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>