drm/vc4: plane: check drm_gem_plane_helper_prepare_fb() return value

Bubble up any error to the caller.

Signed-off-by: Simon Ser <contact@emersion.fr>
Cc: Maxime Ripard <mripard@kernel.org>
Cc: Kees Cook <keescook@chromium.org>
Cc: Dave Stevenson <dave.stevenson@raspberrypi.com>
Reviewed-by: Maíra Canal <mcanal@igalia.com>
Signed-off-by: Maxime Ripard <mripard@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20231216141518.242811-1-contact@emersion.fr