drm/radeon: drop superflous AGP handling

The object flags created in radeon_ttm_placement_from_domain take care that
we use the correct caching for AGP, this is just superflous.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Dave Airlie <airlied@redhat.com>
Link: https://patchwork.freedesktop.org/patch/384336/?series=80346&rev=1