drm/msm: Fix a6xx GMU shutdown sequence

Commit e812744c

 ("drm: msm: a6xx: Add support for A618") missed
updating the VBIF flush in a6xx_gmu_shutdown and instead
inserted the new sequence into a6xx_pm_suspend along with a redundant
GMU idle.

Move a6xx_bus_clear_pending_transactions to a6xx_gmu.c and use it in
the appropriate place in the shutdown routine and remove the redundant
idle call.

v2: Remove newly unused variable that was triggering a warning

Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>
Reviewed-by: Rob Clark <robdclark@gmail.com>
Fixes: e812744c

 ("drm: msm: a6xx: Add support for A618")
Tested-by: Douglas Anderson <dianders@chromium.org>
Signed-off-by: Rob Clark <robdclark@chromium.org>