drm/gud: fix accidentally deleted IS_ERR() check

drm/gud: fix accidentally deleted IS_ERR() check

During conversion of WARN_ON_ONCE to drm_WARN_ON_ONCE in
commit 2d2f1dc74cfb ("drm: gud: replace WARN_ON/WARN_ON_ONCE with
drm versions"), the IS_ERR check was accidentally removed, breaking
the gud_connector_add_properties() function, as any valid pointer
in state_val would produce an error.

The warning was reported by kernel test robot, and is fixed in this patch.

Fixes: 2d2f1dc74cfb ("drm: gud: replace WARN_ON/WARN_ON_ONCE with drm versions")
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/r/202509212215.c8v3RKmL-lkp@intel.com/
Signed-off-by: Ruben Wauters <rubenru09@aol.com>
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://lore.kernel.org/r/20250922173836.5608-1-rubenru09@aol.com