drm/panthor: Fail job creation when the group is dead

Userspace can use GROUP_SUBMIT errors as a trigger to check the group
state and recreate the group if it became unusable. Make sure we
report an error when the group became unusable.

Changes in v3:
- None

Changes in v2:
- Add R-bs

Fixes: de854881 ("drm/panthor: Add the scheduler logical block")
Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Steven Price <steven.price@arm.com>
Reviewed-by: Liviu Dudau <liviu.dudau@arm.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20241029152912.270346-2-boris.brezillon@collabora.com