drm/atomic: Don't set crtc_state->enable manually

The enable field needs to be kept in sync with the mode_blob field. Call
drm_atomic_set_mode_prop_for_crtc() instead of setting enable to false
in order to dereference the mode blob correctly.

v2:
- Check the return value of drm_atomic_set_mode_prop_for_crtc()
- Drop the num_connectors local variable

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>