drm/radeon/kms: get rid of r600_hdmi_find_free_block

R6xx has routable blocks, but there's nothing wrong in assignment based
on dig_encoder. We didn't really need that algorithm.

Signed-off-by: Rafał Miłecki <zajec5@gmail.com>
Tested-by: Christian König <deathsimple@vodafone.de>
Reviewed-by: Christian König <deathsimple@vodafone.de>
Signed-off-by: Dave Airlie <airlied@redhat.com>