drm/vc4: bo: Fix unused variable warning

Commit 07a2975c

 ("drm/vc4: bo: Fix drmm_mutex_init memory hog")
removed the only use of the ret variable, but didn't remove the
variable itself leading to a unused variable warning.

Remove that variable.

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Fixes: 07a2975c

 ("drm/vc4: bo: Fix drmm_mutex_init memory hog")
Reviewed-by: Maíra Canal <mcanal@igalia.com>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://patchwork.freedesktop.org/patch/msgid/20230113154637.1704116-1-maxime@cerno.tech