drm/i915: Allow sprites to be configured on a disabled pipe

We allow cursors to be set up when the pipe is disabled. Do the same for
sprites as well.

We need to be somewhat careful with the primary disable logic as we
don't want to accidentally enable the primary plane on a disabled pipe.

v2: Skip primary enable/disable and plane registers
    writes on disabled pipe

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>