drm/msm/a6xx: Poll for GBIF unhalt status in hw_init

Some GPUs - particularly A7xx ones - are really really stubborn and
sometimes take a longer-than-expected time to finish unhalting GBIF.

Note that this is not caused by the request a few lines above.

Poll for the unhalt ack to make sure we're not trying to write bits to
an essentially dead GPU that can't receive data on its end of the bus.
Failing to do this will result in inexplicable GMU timeouts or worse.

This is a rather ugly hack which introduces a whole lot of latency.

Tested-by: Neil Armstrong <[email protected]> # on SM8550-QRD
Tested-by: Dmitry Baryshkov <[email protected]> # sm8450
Signed-off-by: Konrad Dybcio <[email protected]>
Patchwork: https://patchwork.freedesktop.org/patch/559292/
Signed-off-by: Rob Clark <[email protected]>

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