drm/radeon: move the IB test after the AGP fallback

Otherwise we won't test if the fallback to PCIe GART really worked.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>