drm/i915: Allow the application to choose the constant addressing mode

The relative-to-general state default is useless as it means having to
rewrite the streaming kernels for each batch. Relative-to-surface is
more useful, as that stream usually needs to be rewritten for each
batch. And absolute addressing mode, vital if you start streaming
state, is also only available by adjusting the register...

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>