drm/msm/a6xx: Skip snapshotting unused GMU buffers

Some buffers are unused on certain sub-generations of a6xx.  So just
skip them.

Signed-off-by: Rob Clark <robdclark@chromium.org>
Reviewed-by: Chia-I Wu <olvaffe@gmail.com>
Reviewed-by: Akhil P Oommen <quic_akhilpo@quicinc.com>
Patchwork: https://patchwork.freedesktop.org/patch/507013/
Link: https://lore.kernel.org/r/20221013225520.371226-3-robdclark@gmail.com

drivers/gpu/drm/msm/adreno/a6xx_gpu_state.c

@@ -813,6 +813,9 @@ static struct msm_gpu_state_bo *a6xx_snapshot_gmu_bo(
 {
 	struct msm_gpu_state_bo *snapshot;
 
+	if (!bo->size)
+		return NULL;
+
 	snapshot = state_kcalloc(a6xx_state, 1, sizeof(*snapshot));
 	if (!snapshot)
 		return NULL;