drm/i915: add debugfs file for swizzling information

This will also come handy for the gen6+ swizzling support, where the
driver is supposed to control swizzling depending upon dram
configuration.

v2: CxDRB3 are 16 bit regs! Noticed by Chris Wilson.

Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-Off-by: Daniel Vetter <daniel.vetter@ffwll.ch>