workqueue: Add support for real-time workers

For use cases such as the DRM scheduler submitting work to the GPU on
behalf of low latency userspace applications, where latter have sufficient
privileges to have had successfully obtained realtime Vulkan global
priority, competing with random background CPU load can create large
latency spikes which gets in the way of a smooth user experience.

For these situations the existing WQ_HIGHPRI does not bring a noticeable
improvement and a stronger hint is needed.

Lets add WQ_RT which creates workers with a SCHED_FIFO scheduling class to
improve this.

We use a minimum priority level since we only care about winning the
contest against normal background CPU load.

With this, a benchmark which measures the GPU submit latency against a
real-time Vulkan queue when faced with CPU saturation, highlights a
dramatic reduction in the 95- and 98-th percentiles:

    us      7.3.0   WQ_RT

   median     30     21
    min        2      3
    max     1038    941
   95pct    2384     49
   98pct    2922     63

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@igalia.com>
Cc: Boris Brezillon <boris.brezillon@collabora.com>
Cc: Chia-I Wu <olv@google.com>
Cc: Liviu Dudau <liviu.dudau@arm.com>
Cc: Matthew Brost <matthew.brost@intel.com>
Cc: Steven Price <steven.price@arm.com>
Cc: Tejun Heo <tj@kernel.org>
Reviewed-by: Liviu Dudau <liviu.dudau@arm.com> # v9
Signed-off-by: Tejun Heo <tj@kernel.org>