drm/i915: Extract vfunc setup from logical ring initializers

Majority of them was duplicated code and only render ring
currently overrides some of them. We can save some lines of
code and also take away the confusion on why bsd2 did not
do the seqno coherency workaround. (VCS2 ring does not exist
on platforms where workaround is needed but that was not
documented in the code.)

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/1452619956-27014-1-git-send-email-tvrtko.ursulin@linux.intel.com