ring.rs: fix MI_USER_INTERRUPT value
Was 0x0200_0000 (same as MI_FLUSH_DW) — GPU never generated interrupts.
Correct Gen7+ value: 0x6200_0000 (MI_USER_INTERRUPT with proper encoding)
MI_FLUSH_DW in flush() now uses proper DWord length encoding (1<<22)
scheme.rs: SETPLANE now forwards primary plane to page_flip
Primary plane (id=3) flips via driver.page_flip()
Overlay/cursor planes get silent no-op (KWin falls back to primary)
Removes EOPNOTSUPP blocker for KWin Wayland compositor
vasilito
bd6e9cd70c