i915g: implement cache flushing

i915g: implement cache flushing

With an extremely dumb strategy. But it's the same i915c employs.

Also improve the hw_atom code slightly by statically specifying the
required batch space. For extremely variably stuff (shaders, constants)
it would probably be better to add a new parameter to the hw_atom->validate
function.

Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>