drm/radeon: make radeon_gpu_is_lockup a per ring function

Different rings have different criteria to test
if they are stuck.

v2: rebased on current drm-next

Signed-off-by: Christian König <deathsimple@vodafone.de>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Jerome Glisse <jglisse@redhat.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>