drm: zte: fix static checker warning on variable 'fmt'

Commit 4e986d37 ("drm: zte: add overlay plane support") introduces
the following static checker warning:

 drivers/gpu/drm/zte/zx_plane.c:170 zx_vl_rsz_setup()
 warn: always true condition '(fmt >= 0) => (0-u32max >= 0)'

Fix it by change 'fmt' type to integer.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Fixes: 4e986d37 ("drm: zte: add overlay plane support")
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Reviewed-by: Sean Paul <seanpaul@chromium.org>
Link: http://patchwork.freedesktop.org/patch/msgid/1487598551-28310-1-git-send-email-shawnguo@kernel.org