drm/i915: Refactor blocking waits

Tidy up the for loops that handle waiting for read/write vs read-only
access.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1470293567-10811-13-git-send-email-chris@chris-wilson.co.uk