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