drm/i915: rename macro parameter(ring) to (engine)

'ring' is an old deprecated term for a GPU engine. Here we make the
terminology more consistent by renaming the 'ring' parameter of lots of
macros that calculate addresses within the MMIO space of an engine.

Signed-off-by: Dave Gordon <david.s.gordon@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1469034967-15840-2-git-send-email-david.s.gordon@intel.com
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>