drm/i915: Disable the BLT on pre-production SNB hardware

It never quite worked despite the numerous workarounds, yet I still see
people trying to use this hardware and filing bug reports. As we no
longer even try to implement the workarounds, since 6a233c78878
(drm/i915/ringbuffer: kill snb blt workaround), simply disable the ring.

v2: Add a message to inform the user about the limited capabilities of
their pre-production hardware.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>