drm: move __OS_HAS_AGP into drm_agpsupport.h

With drm_memory.h gone, there is no header left that uses __OS_HAS_AGP.
Move it into drm_agpsupport.h (which is itself included from drmP.h) to
hide it harder from public eyes.

Signed-off-by: David Herrmann <dh.herrmann@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>