drm/i915: Use RCS flips on Ivybridge+

RCS flips do work on Iybridge+ so long as we can unmask the messages
through DERRMR. However, there are quite a few workarounds mentioned
regarding unmasking more than one event or triggering more than one
message through DERRMR. Those workarounds in principle prevent us from
performing pipelined flips (and asynchronous flips across multiple
planes) and equally apply to the "known good" BCS ring. Given that it
already appears to work, and also appears to work with unmasking all 3
planes at once (and queuing flips across multiple planes), be brave.

Bugzlla: https://bugs.freedesktop.org/show_bug.cgi?id=67600
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Lightly-tested-by: Stephane Marchesin <marchesin@icps.u-strasbg.fr>
Cc: Stephane Marchesin <marchesin@icps.u-strasbg.fr>
Cc: Ben Widawsky <ben@bwidawsk.net>
Tested-by: Stéphane Marchesin <marcheu@chromium.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>