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

drivers/gpu/drm/zte/zx_plane.c

@@ -159,7 +159,7 @@ static void zx_vl_rsz_setup(struct zx_plane *zplane, uint32_t format,
 	void __iomem *rsz = zplane->rsz;
 	u32 src_chroma_w = src_w;
 	u32 src_chroma_h = src_h;
-	u32 fmt;
+	int fmt;
 
 	/* Set up source and destination resolution */
 	zx_writel(rsz + RSZ_SRC_CFG, RSZ_VER(src_h - 1) | RSZ_HOR(src_w - 1));
@@ -203,7 +203,7 @@ static void zx_vl_plane_atomic_update(struct drm_plane *plane,
 	u32 src_x, src_y, src_w, src_h;
 	u32 dst_x, dst_y, dst_w, dst_h;
 	uint32_t format;
-	u32 fmt;
+	int fmt;
 	int num_planes;
 	int i;