drm/sched: Fix passing zero to 'PTR_ERR' warning v2

Fix a static code checker warning.

v2: Drop PTR_ERR_OR_ZERO.

Signed-off-by: Andrey Grodzovsky <andrey.grodzovsky@amd.com>
Reviewed-by: Emily Deng <Emily.Deng@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>