drm/msm: Fix races managing the OOB state for timestamp vs timestamps.

Now that we're not racing with GPU setup, also fix races of timestamps
against other timestamps.  In freedreno CI, we were seeing this path trigger
timeouts on setting the GMU bit, producing:

[drm:_a6xx_gmu_set_oob] *ERROR* Timeout waiting for GMU OOB set GPU_SET: 0x0

and this triggered especially on the first set of tests right after
boot (it's probably easier to lose the race than one might think,
given that we start many tests in parallel, and waiting for NFS to
page in code probably means that lots of tests hit the same point of
screen init at the same time).  As of this patch, the message seems to
have completely gone away.

Signed-off-by: Eric Anholt <eric@anholt.net>
Fixes: 4b565ca5

 ("drm/msm: Add A6XX device support")
Reviewed-by: Jordan Crouse <jcrouse@codeaurora.org>
Signed-off-by: Rob Clark <robdclark@chromium.org>