drm/i915: properly check for pipe count

As suggested by Chris Wilson and Daniel Vetter, this chunk of code can be
simplified with a more simple check.

Also, as noticed by Jesse Barnes, it is worth mentioning that plane is an
enum and num_pipe is an int, so we could be more paranoid here about those
validation checks eventually.

CC: Daniel Vetter <daniel.vetter@ffwll.ch>
CC: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Eugeni Dodonov <eugeni.dodonov@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_display.c

@@ -1980,16 +1980,10 @@ intel_pipe_set_base(struct drm_crtc *crtc, int x, int y,
 		return 0;
 	}
 
-	switch (intel_crtc->plane) {
-	case 0:
-	case 1:
-		break;
-	case 2:
-		if (IS_IVYBRIDGE(dev))
-			break;
-		/* fall through otherwise */
-	default:
-		DRM_ERROR("no plane for crtc\n");
+	if(intel_crtc->plane > dev_priv->num_pipe) {
+		DRM_ERROR("no plane for crtc: plane %d, num_pipes %d\n",
+				intel_crtc->plane,
+				dev_priv->num_pipe);
 		return -EINVAL;
 	}