drm/i915: Introduce intel_plane_alloc()

Pull the common plane+plane_state allocation into a small helper.
Reduces the amount of boilerplate in the plane initialization
functions.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181005125817.22576-9-ville.syrjala@linux.intel.com