drm/msm/mdp5: Use runtime PM get/put API instead of toggling clocks

mdp5_enable/disable calls are scattered all around in the MDP5 code.
Use the pm_runtime_get/put calls here instead, and populate the
runtime PM suspend/resume ops to manage the clocks.

About the overall design: MDP5 is a child of the top level MDSS
device. MDSS is also the parent to DSI, HDMI and other interfaces. When
we enable MDP5's power domain, we end up enabling MDSS's PD too. It is
only MDSS's PD that actually controlls the GDSC HW. Therefore, calling
runtime_get/put on the MDP5 device is like just requesting a vote to
enable/disable the GDSC.

Functionally, replacing the clock enable/disable calls with the RPM API
can result in the power domain (GDSC) state being toggled if no other
child isn't powered on. This can result in the register context being lost.
We make sure (in future commits) that code paths don't end up configuring
registers and then later lose state, resulting in a bad HW state.

For now, we've replaced each mdp5_enable/disable with runtime_get/put API.
We could optimize things later by removing runtime_get/put calls which
don't really need to be there. This could prevent unnecessary toggling of
the power domain and clocks.

Signed-off-by: Archit Taneja <architt@codeaurora.org>
Signed-off-by: Rob Clark <robdclark@gmail.com>