drm/v3d: clean caches at the end of render jobs on request from user space

Extends the user space ioctl for CL submissions so it can include a request
to flush the cache once the CL execution has completed. Fixes memory
write violation messages reported by the kernel in workloads involving
shader memory writes (SSBOs, shader images, scratch, etc) which sometimes
also lead to GPU resets during Piglit and CTS workloads.

v2: if v3d_job_init() fails we need to kfree() the job instead of
    v3d_job_put() it (Eric Anholt).

v3 (Eric Anholt):
  - Drop _FLAG suffix from the new flag name.
  - Add a new param so userspace can tell whether cache flushing is
    implemented in the kernel.

Signed-off-by: Iago Toral Quiroga <itoral@igalia.com>