accel/ivpu: Flush pending jobs of device's workqueues

Use flush_work() instead of cancel_work_sync() for driver IRQ
workqueues to guarantee that remaining pending work
will be handled.

This resolves two issues that were encountered where a driver was left
in an incorrect state as the bottom-half was canceled:

1. Cancelling context-abort of a job that is still executing and
   is causing translation faults which is going to cause additional TDRs

2. Cancelling bottom-half of a DCT (duty-cycle throttling) request
   which will cause a device to not be adjusted to an external frequency
   request.

Fixes: bc3e5f48 ("accel/ivpu: Use workqueue for IRQ handling")
Signed-off-by: Maciej Falkowski <maciej.falkowski@linux.intel.com>
Reviewed-by: Lizhi Hou <lizhi.hou@amd.com>
Reviewed-by: Jeff Hugo <jeff.hugo@oss.qualcomm.com>
Signed-off-by: Jacek Lawrynowicz <jacek.lawrynowicz@linux.intel.com>
Link: https://lore.kernel.org/r/20250401155755.4049156-1-maciej.falkowski@linux.intel.com