drm/i915: Unify intel_logical_ring_emit and intel_ring_emit

Both perform the same actions with more or less indirection, so just
unify the code.

v2: Add back a few intel_engine_cs locals

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/1469432687-22756-11-git-send-email-chris@chris-wilson.co.uk
Link: http://patchwork.freedesktop.org/patch/msgid/1470174640-18242-1-git-send-email-chris@chris-wilson.co.uk