drm/msm/a6xx: Avoid freeing gmu resources multiple times

The driver checks for gmu->mmio as a sign that the device has been
initialized, however there are failures in probe below the mmio init.
If one of those is hit, mmio will be non-null but freed.

In that case, a6xx_gmu_probe will return an error to a6xx_gpu_init which
will in turn call a6xx_gmu_remove which checks gmu->mmio and tries to free
resources for a second time. This causes a great boom.

Fix this by adding an initialized member to gmu which is set on
successful probe and cleared on removal.

Changes in v2:
- None

Cc: Jordan Crouse <jcrouse@codeaurora.org>
Reviewed-by: Jordan Crouse <jcrouse@codeaurora.org>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20190523171653.138678-1-sean@poorly.run