drm/radeon: use common next_reloc function

This patch eliminates ASIC-specific ***_cs_packet_next_reloc
functions and hooks up the new common function.

Signed-off-by: Ilija Hadzic <ihadzic@research.bell-labs.com>
Reviewed-by: Marek Olšák <maraeo@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>