drm/msm/dpu: ensure device suspend happens during PM sleep

"The PM core always increments the runtime usage counter
before calling the ->suspend() callback and decrements it
after calling the ->resume() callback"

DPU and DSI are managed as runtime devices. When
suspend is triggered, PM core adds a refcount on all the
devices and calls device suspend, since usage count is
already incremented, runtime suspend was not getting called
and it kept the clocks on which resulted in target not
entering into XO shutdown.

Add changes to force suspend on runtime devices during pm sleep.

Changes in v1:
 - Remove unnecessary checks in the function
    _dpu_kms_disable_dpu (Rob Clark).

Changes in v2:
 - Avoid using suspend_late to reset the usagecount
   as suspend_late might not be called during suspend
   call failures (Doug).

Changes in v3:
 - Use force suspend instead of managing device usage_count
   via runtime put and get API's to trigger callbacks (Doug).

Changes in v4:
 - Check the return values of pm_runtime_force_suspend and
   pm_runtime_force_resume API's and pass appropriately (Doug).

Changes in v5:
 - With v4 patch, test cycle has uncovered issues in device resume.

   On bubs: cmd tx failures were seen as SW is sending panel off
   commands when the dsi resources are turned off.

   Upon suspend, DRM driver will issue a NULL composition to the
   dpu, followed by turning off all the HW blocks.

   v5 changes will serialize the NULL commit and resource unwinding
   by handling them under PM prepare and PM complete phases there by
   ensuring that clks are on when panel off commands are being
   processed.

Changes in v6:
- Use drm_mode_config_helper_suspend/resume() instead of legacy API
  drm_atomic_helper_suspend/resume() (Doug).

  Trigger runtime callbacks from the suspend/resume call to turn
  off the resources.

Changes in v7:
- Add "__maybe_unused" to the functions to avoid compilation
  failures. Cleanup unnecessary configs (Doug).

Signed-off-by: Kalyan Thota <kalyan_t@codeaurora.org>
Reviewed-by: Douglas Anderson <dianders@chromium.org>
Signed-off-by: Rob Clark <robdclark@chromium.org>