drm/msm/mdp5: add tracking for clk enable-count

Accessing registers for an unclocked block is an insta-reboot on
snapdragon devices.  So add a bit of logic to track the enable_count so
we can WARN_ON() unclocked register writes.  This makes it much easier
to track down mistakes.

Signed-off-by: Rob Clark <robdclark@gmail.com>