drm/i915: consolidate ring->add_request a bit

They're indentical, so just kill one. Also give the other a prefix to
distinguish it from the gen6+ functions - this add_request function is
not really generic code.

v2: Fixup commit message as noted by Ben Widawsky.

Reviewed-by: Eric Anholt <eric@anholt.net>
Signed-Off-by: Daniel Vetter <daniel.vetter@ffwll.ch>