agp/intel: introduce intel-agp.h header file

Intel definitions have spilled into agp.h. Create a header file for
them and also include it in efficion-agp.c 'cause it needs a few of
them.

Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Eric Anholt <eric@anholt.net>