i915g: rip out ->sw_tiled

i915g: rip out ->sw_tiled

It looks like this was meant to facilitate unfenced access to textures/
color/renderbuffers. It's totally incomplete and fundamentally broken
on a few levels:
- broken: The kernel needs to about every tiled bo to fix up bit17
  swizzling on swap-in.
- unflexible: fenced/unfenced relocs from execbuffer2 do the same, much
  simpler.
- unneeded: with relaxed fencing tiled gem bos are as memory-efficient
  as this trick.

Hence kill it.

Reviewed-by: Jakob Bornecrantz <wallbraker@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Jakob Bornecrantz <wallbraker@gmail.com>