drm/i915/execlists: Reinitialise context image after GPU hang

On Braswell, at least, we observe that the context image is written in
multiple phases. The first phase is to clear the register state, and
subsequently rewrite it. A GPU reset at the right moment can interrupt
the context update leaving it corrupt, and our update of the RING_HEAD
is not sufficient to restart the engine afterwards. To recover, we need
to reset the registers back to their original values. The context state
is lost. What we need is a better mechanism to serialise the reset with
pending flushes from the GPU.

Fixes: 821ed7df6e2a ("drm/i915: Update reset path to fix incomplete requests")
Signed-off-by: Chris Wilson <[email protected]>
Cc: Mika Kuoppala <[email protected]>
Reviewed-by: Mika Kuoppala <[email protected]>
Link: http://patchwork.freedesktop.org/patch/msgid/[email protected]
(cherry picked from commit a3aabe86a3406b9946a4f7707762a833a58dfe9c)
Signed-off-by: Jani Nikula <[email protected]>

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