drm/radeon/kms: add HDMI code for pre-DCE3 R6xx GPUs

Older GPUs are little different, HDMI blocks are not hard-wired, but routable.
We should just find some free HDMI block and route it to choosen encoder. In
case of RS6x0 there is only one HDMI block, we don't enable HDMI on RS6x00 yet
however.

Signed-off-by: Rafał Miłecki <zajec5@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>