drm/i915: add Ivy Bridge page flip support

Use the blit ring for submitting flips since the render ring doesn't
generate flip complete interrupts.

Fixes bugs:

	https://bugs.freedesktop.org/show_bug.cgi?id=38362
	https://bugs.freedesktop.org/show_bug.cgi?id=38392
	https://bugs.freedesktop.org/show_bug.cgi?id=38393



Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Ben Widawsky <ben@bwidawsk.net>
Tested-by: Jian J Zhao <jian.j.zhao@intel.com>
Signed-off-by: Keith Packard <keithp@keithp.com>