drm/amdgpu: Modify GC register access to use _SOC15 macros

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

Using _SOC15 read/write macros ensures that they go
through RLC when flag is enabled.

Signed-off-by: Rohit Khaire <rohit.khaire@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>