drm/radeon/kms: implement display watermark support for evergreen

Improper display watermarks can result in underflow to the display
controllers which can cause flickering or other artifacts.
This patch implements display watermark support and line buffer
allocation for evergreen asics.

Signed-off-by: Alex Deucher <alexdeucher@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>