mesa: use simple_mtx_t for TexMutex (v2)

mesa: use simple_mtx_t for TexMutex (v2)

change mtx_recursive -> mtx_plain, there's no recursive locking

Let's try this again!  This was originally landed as f6abb3445b7f
("mesa: use simple_mtx_t for TexMutex") and then reverted with
781c0eafcf79 ("Revert "mesa: use simple_mtx_t for TexMutex"") because it
broke i965.  Now that i965 is no longer in the tree, we can restore it.

Acked-By: Mike Blumenkrantz <michael.blumenkrantz@gmail.com> (v1)
Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>
Reviewed-by: Kristian H. Kristensen <hoegsberg@google.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/14053>