drm/msm/a6xx: Make GPU destroy a bit safer

If, for whatever reason, we're trying process adreno_runtime_resume()
at the same time that a6xx_destroy() is running then things can go
boom. Specifically adreno_runtime_resume() will eventually call
a6xx_pm_resume() and that may try to resume the gmu.

Let's grab the GMU lock as we're destroying the GMU. That will solve
the race because a6xx_pm_resume() grabs the same lock. That makes the
access of `gmu->initialized` in a6xx_gmu_resume() safe.

We'll also return an error code in a6xx_gmu_resume() if we see that
`gmu->initialized` was false. If this happens we'll bail out of the
rest of a6xx_pm_resume(), which is good because the rest of that
function is also not good to do if we're racing with a6xx_destroy().

Signed-off-by: Douglas Anderson <dianders@chromium.org>
Patchwork: https://patchwork.freedesktop.org/patch/521232/
Link: https://lore.kernel.org/r/20230202104822.1.I0e49003bf4dd1dead9be4a29dbee41f3b1236e48@changeid
Signed-off-by: Rob Clark <robdclark@chromium.org>

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