drm/i915: Name the IPS_PCODE_CONTROL bit

Give a name to the bit which tells pcode to control IPS.

v2: Note that IPS_CTL bits apply to DISPLAY_IPS_CONTROL as well (Chris)

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170912153411.20171-2-ville.syrjala@linux.intel.com
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>