i915g: fix transfer coherency

i915g: fix transfer coherency

The kernel drm takes care of all coherency as long as we don't forget
to submit all outstanding commands in the batchbuffer ...

Also move batchbuffer initialization up because otherwise transfers
for some helper textures fail with a segmentation fault.

And kill the dead code, flushes should now be correct everywhere.

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