drm/msm: fix fallout of atomic dpms changes

As a result of atomic DPMS support, the various prepare/commit hooks get
called in a way that msm dislikes.  We were expecting prepare/commit to
bracket a modeset, which is no longer the case.  This was needed to hold
various extra clk's (such as interface clks) on while we are touching
registers, and in the case of mdp4 holding vblank enabled.

The most straightforward way to deal with this, since we already have
our own atomic_commit(), is to just handle prepare/commit internally to
the driver (with some additional vfuncs for mdp4 vs mdp5), and switch
everything over to instead use the new enable/disable hooks.  It doesn't
really change too much, despite the code motion.  What used to be in the
encoder/crtc dpms() fxns is split out into enable/disable.

We should be able to drop our own enable-state tracking, as the atomic
helpers should do this for us.  But keeping that for the short term for
extra debugging as atomic stablizes.

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