drm/vc4: remove unneeded variable: "ret"

remove unneeded variable: "ret".

Signed-off-by: Bernard Zhao <bernard@vivo.com>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://patchwork.freedesktop.org/patch/msgid/20210202122338.15351-1-bernard@vivo.com