The hardware has a set of '_NEXT' registers that can hold a second job
while the first is executing. Make use of these registers to enqueue a
second job per slot.

v5:
* Fix a comment in panfrost_job_init()

v3:
* Fix the done/err job dequeuing logic to get a valid active state
* Only enable the second slot on GPUs supporting jobchain disambiguation
* Split interrupt handling in sub-functions
Signed-off-by: Steven Price <steven.price@arm.com>
Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Steven Price <steven.price@arm.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210630062751.2832545-16-boris.brezillon@collabora.com