Similar to what was done for Wayland in 58f90fd03f:
the glthread unmarhsal thread needs to be idle to avoid
concurrent calls to get_back_bo.
Also the existing code flushed after setting dri2_surf->back
to NULL so a new back buffer was always allocated by the
glthread flush:
|---------------> dri2_drm_swap_buffers
| get_back_bo (back=0x55eb93c6c488) > # First get_back_bo call
| get_back_bo (back=0x55eb93c6c488 age: 0)<
| # dri2_surf->back = NULL
|-----> FLUSH
| get_back_bo (back=nil) > # Another get_back_bo call
| get_back_bo (back=0x55eb93c6c4c8 age: 3)<
|-----< FLUSH
|---------------< dri2_drm_swap_buffers
Closes: https://gitlab.freedesktop.org/mesa/mesa/-/issues/10437
Cc: mesa-stable
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/27143>
(cherry picked from commit 6f47e87a60)