drm/i915: move the suspend/resume register file out of dev_priv

dev_priv has grown way too big, and grouping memebers into substructs
and moving them out of line helps re-gain some overview.

Unfortunatley I couldn't just call the substruct save and drop the prefix, since
that will make most member names clash with registers #defines. Changes in
i915_drv.h done by hand, everything else changed with
s/\<save\([A-Z]*\)/regfile.save\1/ in vim.

Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>