intel: Support passing of explicit fencing from execbuf

Allow the caller to pass in an fd to an array of fences to control
serialisation of the execbuf in the kernel and on the GPU, and in return
allow creation of a fence fd for signaling the completion (and flushing)
of the batch. When the returned fence is signaled, all writes to the
buffers inside the batch will be complete and coherent from the cpu, or
other consumers. The return fence is a sync_file object and can be
passed to other users (such as atomic modesetting, or other drivers).

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>