Selecting the wrong or no CONFIG_AGP_* chipset can cause a NULL pointer
dereference when combined with CONFIG_DRM_RADEON_KMS and an old system
with a R100 AGP card (should effect other cards too).  The agp field
will be set to NULL if no suitable AGP chipset driver is loaded,
drm_agp_acquire already preforms a suitable NULL check so it can be used
directly.
Signed-off-by: NRobert Fitzsimons <robfitz@273k.net>
Signed-off-by: NDave Airlie <airlied@redhat.com>