drm: rcar-du: Move plane commit code from CRTC start to CRTC resume

As the DRM core will commit plane states when performing atomic updates,
those don't need to be committed manually when the CRTC is started except
in the system resume code path.

However, the atomic plane commit step is currently performed between
mode set disable and mode set enable to mimick the legacy mode setting
operations order. This causes the device clocks to be disabled after
applying plane settings and reenabled when enabling the CRTC,
potentially losing hardware in between.

Reorder the operations to enable the CRTC first and only then apply
plane settings, removing the need to manage clocks in the atomic begin
and flush handlers. We can then move the plane state commit code out of
the CRTC start handler to the system resume handler.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>