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.

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