agp/intel: split out the GTT support

intel-agp.c contains actually two different drivers: An agp driver
for _physical_ agp slots an the gtt driver that is used by the intel
drm modules. Split them to prevent any further confusion.

This patch just moves the code and includes intel-gtt.c in intel-agp.c
Later patches will untangle these two drivers further.
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Eric Anholt <eric@anholt.net>