drm/i915: Only run commit when crtc is active, v2.

The crtc->active guards are no longer needed now that all state
updates are outside the commit.

Changes since v1:
- Only check crtc->state->active before calling commit_planes_on_crtc.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>