drm/radeon/kms: adjust evergreen display watermark setup

This patch fixes two issues:
- A disabled crtc does not use any lb, so return 0 for
lb size.  This makes the display priority calculation
more exact.
- Only use 1/2 and whole lb partitions. Using smaller
partitions can cause underflow to one of the displays
if you have multiple large displays on the same lb.

Fixes:
https://bugs.freedesktop.org/show_bug.cgi?id=34534



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