drm/sched: Fix static checker warning for potential NULL ptr

Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Andrey Grodzovsky <andrey.grodzovsky@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1558533443-7795-1-git-send-email-andrey.grodzovsky@amd.com