agp: add chipset flushing support to AGP interface

This bumps the AGP interface to 0.103.

Certain Intel chipsets contains a global write buffer, and this can require
flushing from the drm or X.org to make sure all data has hit RAM before
initiating a GPU transfer, due to a lack of coherency with the integrated
graphics device and this buffer.

This just adds generic support to the AGP interfaces, a follow-on patch
will add support to the Intel driver to use this interface.

Signed-off-by: Dave Airlie <airlied@redhat.com>