drm/i915: Move toggling planes out of crtc enable/disable.

This makes disabling planes more explicit.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
[anderco: fixed warning due to using drm_crtc instead of intel_crtc]
Signed-off-by: Ander Conselvan de Oliveira <ander.conselvan.de.oliveira@intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>