drm/i915: Split out GTT specific header file

This file contains all necessary defines, prototypes and typesdefs for
manipulating GEN graphics address translation (this does not include the
legacy AGP driver)

Reiterating the comment in the header,
"Please try to maintain the following order within this file unless it
makes sense to do otherwise. From top to bottom:
1. typedefs
2. #defines, and macros
3. structure definitions
4. function prototypes

Within each section, please try to order by generation in ascending
order, from top to bottom (ie. GEN6 on the top, GEN8 on the bottom)."

I've made some minor cleanups, and fixed a couple of typos while here -
but there should be no functional changes.

The purpose of the patch is to reduce clutter in our main header file,
making room for new growth, and make documentation of our interfaces
easier by splitting things out.

With a little more work, like making i915_gtt a pointer, we could
potentially completely isolate this header from i915_drv.h. At the
moment however, I don't think it's worth the effort.

Personally, I would have liked to put the PTE encoding functions in this
file too, but I didn't want to rock the boat too much.

A similar patch has been in use on my machine for some time. This exact
patch though has only been compile tested.

Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>