drm/radeon/cik: plug in missing blit callback

I implemented support for this, but forget to hook
up the callback so the driver can actually use it.
On asics with a dedicated DMA engine, we use the DMA
engine for buffer migration so this is just for testing
purposes.

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

diff --cc drivers/gpu/drm/radeon/radeon_asic.c
Simple merge