drm/panfrost: Make ->run_job() return an ERR_PTR() when appropriate

If the fence creation fail, we can return the error pointer directly.
The core will update the fence error accordingly.

Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Steven Price <steven.price@arm.com>
Reviewed-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210630062751.2832545-4-boris.brezillon@collabora.com