drm/gud: fix sizeof use

drivers/gpu/drm/gud/gud_connector.c:710:37-43: ERROR: application of sizeof to pointer

 sizeof when applied to a pointer typed expression gives the size of
 the pointer

Generated by: scripts/coccinelle/misc/noderef.cocci

Fixes: 40e1a70b

 ("drm: Add GUD USB Display driver")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: kernel test robot <lkp@intel.com>
[fix subject]
Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20210322174434.58849-1-noralf@tronnes.org