drm/i915: move the wait_rendering call into flush_gpu_write_domain

One caller (for the pageflip support) wants a purely pipelined flush.
Distinguish this case by a new parameter. This will also be useful
later on for pipelined fencing.

v2: Simplify the code by depending upon the implicit request emitting
of i915_wait_request.

Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
[ickle: And drop the non-interruptible support in the process.]
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>