drm/msm/mdp5: Set up runtime PM for MDSS

MDSS represents the top level wrapper that contains MDP5, DSI, HDMI and
other sub-blocks. W.r.t device heirarchy, it's the parent of all these
devices. The power domain of this device is actually tied to the GDSC
hw. When any sub-device enables its PD, MDSS's PD is also enabled.

The suspend/resume ops enable the top level clocks that end at the MDSS
boundary. For now, we're letting them all be optional, since the child
devices anyway hold a ref to these clocks.

Until now, we'd called a runtime_get() during probe, which ensured that
the GDSC was always on. Now that we've set up runtime PM for the children
devices, we can get rid of this hack.

Note: that the MDSS device is the platform_device in msm_drv.c. The
msm_runtime_suspend/resume ops call the funcs that enable/disable
the top level MDSS clocks. This is different from MDP4, where the
platform device created in msm_drv.c represents MDP4 itself. It would
have been nicer to hide these differences by adding new kms funcs, but
runtime PM needs to be enabled before kms is set up (i.e, msm_kms_init
is called).

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