drm/plane: Fix IS_ERR() vs NULL check in drm_plane_create_hotspot_properties()

drm/plane: Fix IS_ERR() vs NULL check in drm_plane_create_hotspot_properties()

The drm_property_create_signed_range() function doesn't return error
pointers it returns NULL on error.  Fix the error checking to match.

Fixes: 8f7179a1027d ("drm/atomic: Add support for mouse hotspots")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Reviewed-by: Zack Rusin <zack.rusin@broadcom.com>
Link: https://patch.msgid.link/aTB023cfcIPkCsFS@stanley.mountain
Signed-off-by: Maxime Ripard <mripard@kernel.org>