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>

drivers/gpu/drm/amd/amdgpu/amdgpu_device.c

@@ -719,7 +719,7 @@ bool amdgpu_need_post(struct amdgpu_device *adev)
 	/* then check MEM_SIZE, in case the crtcs are off */
 	reg = amdgpu_asic_get_config_memsize(adev);
 
-	if (reg)
+	if ((reg != 0) && (reg != 0xffffffff))
 		return false;
 
 	return true;