drm/amdgpu: refine the logic in amdgpu_need_post()

We check the mem config register to make sure it's been
programmed by the vbios to determine if we need to post
so we check for a non-0 value.  However, when the asic
comes out of reset, we may see all ones here, so check
for that too.

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