drm/i915: check fb->pixel_format instead of bits_per_pixel

We've mostly switched over to the new more flexible schema, but
there's one check left in the modeset code.

Motivated by a question from Ville whether there's really no way an
unsupported pixel_format can escape into our platform update_plane
callbacks.

v2: Ville noticed that the fb->depth check is redudant when we already
check fb->pixel_format.

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_display.c

@@ -7954,10 +7954,8 @@ intel_set_config_compute_mode_changes(struct drm_mode_set *set,
 			config->mode_changed = true;
 		} else if (set->fb == NULL) {
 			config->mode_changed = true;
-		} else if (set->fb->depth != set->crtc->fb->depth) {
-			config->mode_changed = true;
-		} else if (set->fb->bits_per_pixel !=
-			   set->crtc->fb->bits_per_pixel) {
+		} else if (set->fb->pixel_format !=
+			   set->crtc->fb->pixel_format) {
 			config->mode_changed = true;
 		} else
 			config->fb_changed = true;