backend-drm: make sure outputs are enabled during plane assignment

drm_output_propose_state() is called during from drm_assign_planes(). At this
point, the output is enabled, but it may be disabled in the current state, that
is copied.

So explicitly enable the output by setting dpms to WESTON_DPMS_ON.
The same thing is already done in drm_output_repaint() but that's to late for
drm_output_propose_state(). Move that to the fallback path when planes are
disabled. It's no longer needed in general and changing the state between the
atomic test and commit is not a good idea anyways.

Without this, the atomic commit may fail, even though the corresponding test was
successful.

Signed-off-by: Michael Olbrich <m.olbrich@pengutronix.de>