drm/i915: Remove implicitly disabling primary plane for now

Some of the flags that were used are still useful when transitioning
to atomic, so keep those around for now. This removes some of the
complications of crtc->primary_enabled, making it easier to remove.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Ander Conselvan de Oliveira <ander.conselvan.de.oliveira@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>