Make the code self-documenting by introducing i9xx_has_pfit().
Also make PNV an exceptional case so that we can unset
.is_mobile for the desktop variant.

v2: s/gen4/gen>=4/ (Tvrtko)

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/20190319142329.22881-1-ville.syrjala@linux.intel.comReviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>