drm/kms/radeon/agp: Move the check of the aper_size after drm_acp_acquire and drm_agp_info

First call drm_agp_acquire to check if agp has been acquired.
Second call drm_agp_info to fill in the info data struct, including aper_size.
Finally do the check to see if the aper_size makes sense.

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