drm/amdgpu: read gfx register using RREG32_KIQ macro

Reading CP_MEM_SLP_CNTL register with RREG32_SOC15 macro will lead to
hang when GPU is in "gfxoff" state.
I do a uniform substitution here.

Signed-off-by: chen gong <curry.gong@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>