drm/vc4: Handing the return value of drm_universal_plane_init

Handing the return value of drm_universal_plane_init to fix the following
W=1 kernel build warning(s):
vc4_plane.c: In function ‘vc4_plane_init’:
vc4_plane.c:1340:6: warning: variable ‘ret’ set but not
used [-Wunused-but-set-variable]

Signed-off-by: Tian Tao <tiantao6@hisilicon.com>
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/1599811777-34093-1-git-send-email-tiantao6@hisilicon.com