drm/i915: Avoid div-by-zero when computing aux_stride w/o an aux plane

To make life easier let's allow skl_plane_stride() to be called for the
AUX surface even when there is no AUX surface. Avoids special cases in
the callers.
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170307194210.13400-4-ville.syrjala@linux.intel.com

drivers/gpu/drm/i915/intel_display.c

@@ -3218,7 +3218,12 @@ static void skl_detach_scalers(struct intel_crtc *intel_crtc)
 u32 skl_plane_stride(const struct drm_framebuffer *fb, int plane,
 		     unsigned int rotation)
 {
-	u32 stride = intel_fb_pitch(fb, plane, rotation);
+	u32 stride;
+
+	if (plane >= fb->format->num_planes)
+		return 0;
+
+	stride = intel_fb_pitch(fb, plane, rotation);
 
 	/*
 	 * The stride is either expressed as a multiple of 64 bytes chunks for