drm/amdgpu: Use RLCG to program GRBM_GFX_CNTL during full access time

[Why]
KIQ register init requires GRBM_GFX_CNTL to select KIQ.

[How]
As RLCG accessing registers will save the data of GRBM_GFX_CNTL and restore it.
Use RLCG indirect accessing register method to select grbm instead of mmio directly access.

Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Yifan Zha <Yifan.Zha@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>