drm/amdgpu: check scratch registers to see if we need post (v2)

Rather than checking the CONGIG_MEMSIZE register as that may
not be reliable on some APUs.

v2: The scratch register is only used on CIK+

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