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