drm/radeon/kms: clean assigning HDMI blocks to encoders

We almost always used first HDMI block for first encoder and second for sencod.
Exception was KLDSCP_LVTMA. Analyzing code picking DIG encoder shows the same
behaviour. It shows HDMI block are related to DIGs, which relation we now use.

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