drm/i915: Remove intel_ring_get_tail()

Joonas doesn't like the tiny function, especially if I go around making
it more complicated and using it elsewhere. To remove that temptation,
remove the function!

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/1469432687-22756-21-git-send-email-chris@chris-wilson.co.uk


Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1470174640-18242-11-git-send-email-chris@chris-wilson.co.uk