backend-drm: Don't force non-opaque overlays to primary plane

If a view is non-opaque - such as an overlay over a video - we shouldn't
force it to be on the primary plane, as that's where the underlying
content should be placed, such as the video view.

dc0de9ee already mentioned: "This check should be changed in future to
only filter for opaque views, but that's for another time."

Adding "Fixes" at this is arguably a bug fix:
Fixes: dc0de9ee (backend-drm: Move overlay vs. primary plane check earlier)
Fixes: 2538aacc (backend-drm: Construct a zpos candidate list of planes)

Signed-off-by: Robert Mader <robert.mader@collabora.com>