Fix mixed-mode/renderer-only mode for HW plane assignment of views

These fix a few issues we've seen seeing but also reported by @ofourdan in #304 (closed)

• incorrectly detected zpos values when overlay TEST_ONLY failed - so we skip testing planes that aren't enabled and assert() if we detect any invalid zpos value
• the other plane that was having a duplicate values was the primary which contained the same invalid zpos value: we now assign zpos_min to the primary plane switching to mixed-mode of compositing
• we still tried HW plane assignment of views when switching mixed-mode to renderer-only. This check was removed in 2538aacc so we add it back in drm_output_prepare_plane_view().
• change to a (hard) assert when detecting duplicated zpos values
• lastly, renderer-only requires sets spirtes_are_broken so avoid any overlay assignment whenever that debug keybinding is on.

/cc @daniels /cc @ofourdan