drm/i915/execlists: Use rmb() to order CSB reads

We assume that the CSB is written using the normal ringbuffer
coherency protocols, as outlined in kernel/events/ring_buffer.c:

    *   (HW)                              (DRIVER)
    *
    *   if (LOAD ->data_tail) {            LOAD ->data_head
    *                      (A)             smp_rmb()       (C)
    *      STORE $data                     LOAD $data
    *      smp_wmb()       (B)             smp_mb()        (D)
    *      STORE ->data_head               STORE ->data_tail
    *   }

So we assume that the HW fulfils its ordering requirements (B), and so
we should use a complimentary rmb (C) to ensure that our read of its
WRITE pointer is completed before we start accessing the data.

The final mb (D) is implied by the uncached mmio we perform to inform
the HW of our READ pointer.

References: https://bugs.freedesktop.org/show_bug.cgi?id=105064
References: https://bugs.freedesktop.org/show_bug.cgi?id=105888
References: https://bugs.freedesktop.org/show_bug.cgi?id=106185
Fixes: 767a983ab255 ("drm/i915/execlists: Read the context-status HEAD from the HWSP")
References: 61bf9719fa17 ("drm/i915/cnl: Use mmio access to context status buffer")
Suggested-by: Mika Kuoppala <[email protected]>
Signed-off-by: Chris Wilson <[email protected]>
Cc: Mika Kuoppala <[email protected]>
Cc: Joonas Lahtinen <[email protected]>
Cc: Tvrtko Ursulin <[email protected]>
Cc: Michał Winiarski <[email protected]>
Cc: Rafael Antognolli <[email protected]>
Cc: Michel Thierry <[email protected]>
Cc: Timo Aaltonen <[email protected]>
Tested-by: Timo Aaltonen <[email protected]>
Acked-by: Michel Thierry <[email protected]>
Acked-by: Mika Kuoppala <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]

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