drm: move AGP definitions harder

Move drm_agp_head to drm_agpsupport.h and drm_agp_mem into drm_legacy.h.
Unfortunately, drivers still heavily access drm_agp_head so we cannot
move it to drm_legacy.h. However, at least it's no longer visible in
drmP.h now (it's directly included from it, though).

Signed-off-by: David Herrmann <dh.herrmann@gmail.com>
Reviewed-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>