drm/imx/dcss: fix unused but set variable warnings

Fix unused but set variable warning building with `make W=1`:

drivers/gpu/drm/imx/dcss/dcss-plane.c:270:6: warning:
 variable ‘pixel_format’ set but not used [-Wunused-but-set-variable]
  u32 pixel_format;
      ^~~~~~~~~~~~

Fixes: 9021c317 ("drm/imx: Add initial support for DCSS on iMX8MQ")
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Wang ShaoBo <bobo.shaobowang@huawei.com>
Reviewed-by: Laurentiu Palcu <laurentiu.palcu@oss.nxp.com>
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20200911014414.4663-1-bobo.shaobowang@huawei.com

drivers/gpu/drm/imx/dcss/dcss-plane.c

@@ -267,7 +267,6 @@ static void dcss_plane_atomic_update(struct drm_plane *plane,
 	struct dcss_plane *dcss_plane = to_dcss_plane(plane);
 	struct dcss_dev *dcss = plane->dev->dev_private;
 	struct drm_framebuffer *fb = state->fb;
-	u32 pixel_format;
 	struct drm_crtc_state *crtc_state;
 	bool modifiers_present;
 	u32 src_w, src_h, dst_w, dst_h;
@@ -277,7 +276,6 @@ static void dcss_plane_atomic_update(struct drm_plane *plane,
 	if (!fb || !state->crtc || !state->visible)
 		return;
 
-	pixel_format = state->fb->format->format;
 	crtc_state = state->crtc->state;
 	modifiers_present = !!(fb->flags & DRM_MODE_FB_MODIFIERS);