drm: Change create block to reserve node

With the previous patch we no longer actually create a node, we simply
find the correct hole and occupy it. This very well could have been
squashed with the last patch, but since I already had David's review, I
figured it's easiest to keep it distinct.

Also update the users in i915. Conveniently this is the only user of the
interface.

CC: David Airlie <airlied@linux.ie>
CC: <dri-devel@lists.freedesktop.org>
Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Acked-by: David Airlie <airlied@linux.ie>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>