drm/i915: Add a name for the Punit GPLLENABLE bit

Remove the magic number for the GPLLENABLE bit by adding a name for it.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Deepak <S&lt;deepak.s@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>