drm/radeon/kms: Disable AGP is aperture size < 32M

radeon KMS need a GART of at least 32M to properly work. This patch
check the AGP aperture size and disable if it's less than 32M. Note
than unlike non KMS path we don't staticaly allocate AGP memory so
we are not wasting memory not used by graphic processing.

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