drm/i915: Add aliases for uapi and hw to plane_state

Prepare to split up hw and uapi machinally, by adding a uapi and
hw alias. We will remove the base in a bit. This is a split from the
original uapi/hw patch, which did it all in one go.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191031112610.27608-7-maarten.lankhorst@linux.intel.com
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>