drm/amdgpu: Fix tracking of invalid userptrs

Restore the code that resets mem->invalid. Othewise so mapping
userptrs after they got an MMU notifiers would always be skipped.

This also avoids unnecessarily calling get_user_pages on BOs that
have not been invalidated since the last try.
Signed-off-by: Felix Kuehling <Felix.Kuehling@amd.com>
Reviewed-by: Philip Yang <Philip.Yang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

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

@@ -1740,6 +1740,12 @@ static int update_invalid_user_pages(struct amdkfd_process_info *process_info,
 		}
 
 		amdgpu_ttm_tt_get_user_pages_done(bo->tbo.ttm);
+
+		/* Mark the BO as valid unless it was invalidated
+		 * again concurrently.
+		 */
+		if (atomic_cmpxchg(&mem->invalid, invalid, 0) != invalid)
+			return -EAGAIN;
 	}
 
 	return 0;