drm/i915: s/IS_G4X && !IS_GM45/IS_G45/

We have IS_G45 these days. Let's use it instead of the
'IS_G4X && !IS_GM45' construct.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180614180500.2565-1-ville.syrjala@linux.intel.com
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>