backend-drm: Use plane's zpos_min to check for underlay ability

Previously, whether a KMS plane is underlay-able is determined by whether it's zpos_max is < the primary plane's zpos_min. In other words, a plane will only be considered underlay-able if its entire valid zpos range is under the primary plane's lowest zpos.

This is too restrictive - it's possible for planes to have a valid zpos range that spans below and above the primary's zpos range.

Therefore, allow planes to be used as underlays if their zpos_min is < the primary plane's zpos_min.

An enum is also added for determining whether the plane type is underlay-only, overlay-only, or both. Currently, this is only used for printing better drm-backend logs.

UPDATE: Instead of marking a plane's type before it's assignment, we consider all overlay planes as "secondary planes" at first. Whether it's assigned as an overlay or underlay is printed in the drm backend logs after it gets assigned.

Signed-off-by: Leo Li sunpeng.li@amd.com