Fix issue #9 ("panic: <something>: called with spinlock or critical section held" on KabyLake)

[wulf7]

This fixes several panics triggered by running of fence callbacks in critical section.

It includes:

1. irq_work implementation via fast taskqueue: https://reviews.freebsd.org/D27171. Panic message is in review
2. SYSINIT initialization of current for irq_work thread as it can not be done lazily by i915kms without extra patching.
3. Asynchronous kfree() to use inside critical sections. This fixes panic from "panic: <something>: called with spinlock or critical section held" on KabyLake #9

It is possible that p. 2 and 3 should belong to LKPI rather than to drm-kmod.

[0mp]

Potentially related to https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=250772.

[evadot]

Pushed with some modification (check was wrong in i915_request.c)
Thanks !

[0mp]

Thanks, @evadot!