drm/i915: remove !enabled handling from commit primary plane step

The !crtc->enabled case will now be handled by the !visible code,
since the handling is basically the same.

Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>