drm/i915/ringbuffer: kill snb blt workaround

This was just to facilitate product enablement with pre-production hw.
Allows us to kill quite a bit of cruft.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>