drm/i915: No panel fitter on 830M or non-mobile gen2/3 platforms

PFIT_CONTROL doesn't exist on 830M, so avoid reading it in
i9xx_get_pfit_config().

Also assume that only mobile gen2/3 chipsets have a panel fitter. This
matches the documentation, but I didn't have real hardware to verify.

Gen4 docmentation is a bit inconsistent, but experimenetation on my
LPT machine suggests that the panel fitter is available on non-mobile
gen4 platforms. At least on this machine panel fitter appears works
just fine even on VGA output.
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_display.c

@@ -5439,6 +5439,9 @@ static void i9xx_get_pfit_config(struct intel_crtc *crtc,
 	struct drm_i915_private *dev_priv = dev->dev_private;
 	uint32_t tmp;
 
+	if (INTEL_INFO(dev)->gen <= 3 && (IS_I830(dev) || !IS_MOBILE(dev)))
+		return;
+
 	tmp = I915_READ(PFIT_CONTROL);
 	if (!(tmp & PFIT_ENABLE))
 		return;

drivers/gpu/drm/i915/intel_overlay.c

@@ -1005,7 +1005,7 @@ static int intel_panel_fitter_pipe(struct drm_device *dev)
 	u32  pfit_control;
 
 	/* i830 doesn't have a panel fitter */
-	if (IS_I830(dev))
+	if (INTEL_INFO(dev)->gen <= 3 && (IS_I830(dev) || !IS_MOBILE(dev)))
 		return -1;
 
 	pfit_control = I915_READ(PFIT_CONTROL);