drm/i915: Set primary_disabled in intel_{enable, disable}_plane

If the primary gets marked as disabled while the pipe is off for
instance, we should still re-enable it when the pipe is turned on,
unless the sprite covers it fully also in that configuration.
Unfortunately we do the plane visibility checks only in the sprite code,
which is executed after the primary enabling when turning the pipe off.

Ideally we should compute the plane visibility before touching the
hardware at all, but for now just set the primary_disabld flag
in intel_{enable,disable}_plane.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>