drm/i915: Rename ring flush functions

Historically we considered the render ring to have special flush
semantics and everything else to fall under a more general umbrella.
Probably by coincidence more than anything we decided to make the bsd
ring have the default *other* flush. As the new vebox ring exposes, the
bsd ring is actually the weird one. Doing this allows us to call
gen6_ring_flush for the vebox because calling blt_ring_flush would be
weird...

This patch should have no functional change.

Reviewed-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>