drm/i915: Add probe and remove to the gtt ops

The idea, and much of the code came originally from:

commit 0712f0249c3148d8cf42a3703403c278590d4de5
Author: Ben Widawsky <ben@bwidawsk.net>
Date:   Fri Jan 18 17:23:16 2013 -0800

    drm/i915: Create a vtable for i915 gtt

Daniel didn't like the color of that patch series, and so I asked him to
start something which appealed to his sense of color. The preceding
patches are those, and now this is going on top of that.

[extracted from the original commit message]

One immediately obvious thing to implement is our gmch probing. The init
function was getting massively bloated. Fundamentally, all that's needed
from GMCH probing is the GTT size, and the stolen size. It makes design
sense to put the mappable calculation in there as well, but the code
turns out a bit nicer without it (IMO)

The intel_gtt bridge thing is still here, but the subsequent patches
will finish ripping that out.

Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
[danvet: Bikeshedded one comment (GMADR is just the PCI aperture, we
use it for other things than just accessing tiled surfaces through a
linear view) and cut the newly added long lines a bit. Also one
checkpatch error.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>