drm/radeon/kms/r600: fix forcing pci mode on agp cards

All we need to do on r6xx/r7xx is clear the RADEON_IS_AGP
flag; the rest is handled in r600.c

fixes fdo bug 23990:
http://bugs.freedesktop.org/show_bug.cgi?id=23990

Signed-off-by: Alex Deucher <alexdeucher@gmail.com>
Signed-off-by: Dave Airlie <airlied@linux.ie>