drm/msm/a6xx: Use rev to identify SKU

Use rev instead of revn to identify the SKU. This is in
preparation to the introduction of 7c3 gpu which won't have a
revn.

Signed-off-by: Akhil P Oommen <akhilpo@codeaurora.org>
Link: https://lore.kernel.org/r/20210730011945.v4.2.I286ef007fcadd9e6ee3b2c0ad948f990735f9610@changeid


Signed-off-by: Rob Clark <robdclark@chromium.org>