drm/i915: Introduce i9xx_has_pps()

Add a small helper to determine if we have the panel power
sequencer or not. We'll make PNV an exceptional case so
that we can unset .is_mobile for the desktop variant.

Cc: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190318165633.28924-3-ville.syrjala@linux.intel.com


Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>