Fixes gcc '-Wunused-but-set-variable' warning:

drivers/gpu/drm/vc4/vc4_txp.c: In function 'vc4_txp_connector_atomic_check':
drivers/gpu/drm/vc4/vc4_txp.c:252:29: warning:
 variable 'gem' set but not used [-Wunused-but-set-variable]
  struct drm_gem_cma_object *gem;
                             ^
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Eric Anholt <eric@anholt.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20190215033507.103232-1-yuehaibing@huawei.comSigned-off-by: Maxime Ripard <maxime.ripard@bootlin.com>