drm/i915/bdw: Render ring flushing

PIPE_CONTROL added the high address dword. I'm not sure how the
simulator let me get away with this. I've explicitly left out all the
workarounds from Gen7 because in the minimal digging that I did, most
don't seem necessary, and the simulator doesn't complain without them

Note that BLT and BSD ring commands had already been updated previously.
Just render/pipe_control should have been broken.

v2: Squash in a fixup from Ville to follow the recent IVB PIPE_CONTROL
updates: "BDW uses the IVB PIPE_CONTROL style for specifying GTT vs.
PPGTT for the PIPE_CONTROL QW/DW write."

v3: Rebase on top of Chris' cleanup to have an explicit ring->scratch
buffer object instead of an opaque ring->private where everyone stores
the same stuff inside.

Reported-by: Damien Lespiau <damien.lespiau@intel.com>
Reviewed-by: Ben Widawsky <ben@bwidawsk.net> (for the fixup)
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Ben Widawsky <ben@bwidawsk.net> (v1)
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>