drm/i915: implement drmmode overlay support v4

This implements intel overlay support for kms via a device-specific
ioctl. Thomas Hellstrom brought up the idea of a general ioctl (on
dri-devel). We've reached the conclusion that such an infrastructure
only makes sense when multiple kms overlay implementations exists,
which atm don't (and it doesn't look like this is gonna change).

Open issues:
- Runs in sync with the gpu, i.e. unnecessary waiting. I've decided
  to wait on this because the hw tends to hang when changing something
  in this area. I left some dummy functions as infrastructure.
- polyphase filtering uses a static table.
- uses uninterruptible sleeps. Unfortunately the alternatives may
  unnecessarily wedged the hw if/when we timeout too early (and
  userspace only overloaded the batch buffers with stuff worth a few
  secs of gpu time).

Changes since v1:
- fix off-by-one misconception on my side. This fixes fullscreen
  playback.
Changes since v2:
- add underrun detection as spec'ed for i965.
- flush caches properly, fixing visual corruptions.
Changes since v4:
- fix up cache flushing of overlay memory regs.
- killed require_pipe_a logic - it hangs the chip.

Tested-By: diego.abelenda@gmail.com (on a 865G)
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
[anholt: Resolved against the MADVISE ioctl going in before this one]
Signed-off-by: Eric Anholt <eric@anholt.net>