drm/radeon/kms: keep HDMI state in separated variable

If we want hdmi_offset to be relative to the first block, zero value can
be used also for enabled block.

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>