i965/sync: Fix uninitalized usage and leak of mutex

i965/sync: Fix uninitalized usage and leak of mutex

We locked an unitialized mutex in the callstack
    glClientWaitSync
    intel_gl_client_wait_sync
    brw_fence_client_wait_sync
because we forgot to initialize it in intel_gl_fence_sync.
(The EGLSync codepath didn't have this bug. It initialized the mutex in
intel_dri_create_sync).

We also forgot to tear down (mtx_destroy) the mutex when destroying
the sync object.

Cc: mesa-stable@lists.freedesktop.org
Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>