i965: store reference to the context within struct brw_fence (v2)

i965: store reference to the context within struct brw_fence (v2)

As the spec allows for {server,client}_wait_sync to be called without
currently bound context, while our implementation requires context
pointer.

v2: Add a mutex and acquire it for the duration of
    brw_fence_client_wait() and brw_fence_is_completed() as suggested
    by Chad.

Cc: "11.2 12.0" <mesa-stable@lists.freedesktop.org>
Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Signed-off-by: Tomasz Figa <tfiga@chromium.org>