gallium/util: Increase the debug_flush map depth

gallium/util: Increase the debug_flush map depth

Some piglit tests trigger a map depth assert when debug_flush is active.
Fix this by increasing the map depth from 16 to 32.

Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
Reviewed-by: Charmaine Lee <charmainel@vmware.com>
Reviewed-by: Brian Paul <brianp@vmware.com>
Reviewed-by: Roland Scheidegger <sroland@vmware.com>
Tested-by: Marge Bot <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3614>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3614>