drm/i915: Use LRI for switching PP_DIR_BASE

The docs seem to suggest this is the appropriate method (though it
doesn't say so outright). In other words, we probably should have done
this before. We certainly must do this for switching VMs on the fly,
since synchronizing the rings to MMIO updates isn't acceptable.

v2:
Make the reset code actually work for all rings. Note that this was
fixed in subsequent commits, but was indeed broken for this commit.

Add a posting read to the reset case. It probably should have existed
before hand, but since we have no failures; there is no reason to make
it a separate commit.

Make IS_GEN6 not use the ring because I am seeing crashes when using it.
It is a bit of a hack in this patch, it will get fixed up in a couple of
patches.

Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>