drm/vc4: kms: Wait for the commit before increasing our clock rate

Several DRM/KMS atomic commits can run in parallel if they affect
different CRTC. These commits share the global HVS state, so we have
some code to make sure we run commits in sequence. This synchronization
code is one of the first thing that runs in vc4_atomic_commit_tail().

Another constraints we have is that we need to make sure the HVS clock
gets a boost during the commit. That code relies on clk_set_min_rate and
will remove the old minimum and set a new one. We also need another,
temporary, minimum for the duration of the commit.

The algorithm is thus to set a temporary minimum, drop the previous
one, do the commit, and finally set the minimum for the current mode.

However, the part that sets the temporary minimum and drops the older
one runs before the commit synchronization code.

Thus, under the proper conditions, we can end up mixing up the minimums
and ending up with the wrong one for our current step.

To avoid it, let's move the clock setup in the protected section.

Fixes: d7d96c00

 ("drm/vc4: hvs: Boost the core clock during modeset")
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Reviewed-by: Dave Stevenson <dave.stevenson@raspberrypi.com>
Tested-by: Jian-Hong Pan <jhp@endlessos.org>
Link: https://lore.kernel.org/r/20211117094527.146275-2-maxime@cerno.tech