drm/omapdrm: Move commit_modeset_enables() before commit_planes()

Move drm_atomic_helper_commit_modeset_enables() call to before
drm_atomic_helper_commit_planes() call and have a
omap_atomic_wait_for_completion() call after both.

With the current dss dispc implementation we have to enable the new
modeset before we can commit planes. The dispc ovl configuration
relies on the video mode configuration been written into the HW when
the ovl configuration is calculated.

This approach is not ideal because after a mode change the plane
update is executed only after the first vblank interrupt. The dispc
implementation should be fixed so that it is able use uncommitted drm
state information.  information.

Signed-off-by: Jyri Sarha <jsarha@ti.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>