drm/1915: ringbuffer wait for idle function

Added a new function which waits for the ringbuffer space to be equal to
(total - 8). This is the empty condition of the ringbuffer, and
equivalent to head==tail.

Also modified two users of this functionality elsewhere in the code.

Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>