drm/i915: Introduce i9xx_has_pfit()

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>

drivers/gpu/drm/i915/intel_display.c

@@ -7962,14 +7962,22 @@ static int vlv_crtc_compute_clock(struct intel_crtc *crtc,
 	return 0;
 }
 
+static bool i9xx_has_pfit(struct drm_i915_private *dev_priv)
+{
+	if (IS_I830(dev_priv))
+		return false;
+
+	return INTEL_GEN(dev_priv) >= 4 ||
+		IS_PINEVIEW(dev_priv) || IS_MOBILE(dev_priv);
+}
+
 static void i9xx_get_pfit_config(struct intel_crtc *crtc,
 				 struct intel_crtc_state *pipe_config)
 {
 	struct drm_i915_private *dev_priv = to_i915(crtc->base.dev);
 	u32 tmp;
 
-	if (INTEL_GEN(dev_priv) <= 3 &&
-	    (IS_I830(dev_priv) || !IS_MOBILE(dev_priv)))
+	if (!i9xx_has_pfit(dev_priv))
 		return;
 
 	tmp = I915_READ(PFIT_CONTROL);