drm/i915: Rework plane readout.

All non-primary planes get disabled during hw readout,
this reduces complexity and means not having to do some plane
visibility checks during the first commit.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Daniel Stone <daniels@collabora.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>