drm/radeon: update line buffer allocation for dce4.1/5

commit 0b31e02363b0db4e7931561bc6c141436e729d9f upstream.

We need to allocate line buffer to each display when
setting up the watermarks.  Failure to do so can lead
to a blank screen.  This fixes blank screen problems
on dce4.1/5 asics.

Based on an initial fix from:
Jay Cornwall <jay.cornwall@amd.com>

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>