RGA: use uninterruptible sleep for ioctls

An interruptible syscall can be interrupted by signals. The
expectation under normal circumstances is that the syscall can be
safely restarted by being called with the same arguments, in which
case it must return -ERESTARTSYS.

By this definition, rga_blit_sync isn't interruptible, since the
task could be interrupted after the request has already been sent
to hardware. Restarting such a request would always result in
significant duplicate work and in the case of overlapped source
and destination it could result in corrupted data. In any case,
rga_blit_sync returned a timeout status for interruptions,
causing what appeared to be spurious errors from the userspace
perspective.

rga_flush could technically be interruptible, but experimentation
showed that restarting this syscall could add significant latency.
For implementation simplicity, rga_flush has also been made
uninterruptible.