drm/msm/dp: wait for hpd high before aux transaction

The source device should ensure the sink is ready before proceeding to
read the sink capability or perform any aux transactions. The sink
will indicate its readiness by asserting the HPD line. The controller
driver needs to wait for the hpd line to be asserted by the sink before
it performs any aux transactions.

The eDP sink is assumed to be always connected. It needs power from the
source and its HPD line will be asserted only after the panel is powered
on. The panel power will be enabled from the panel-edp driver and only
after that, the hpd line will be asserted.

Whereas for DP, the sink can be hotplugged and unplugged anytime. The hpd
line gets asserted to indicate the sink is connected and ready. Hence
there is no need to wait for the hpd line to be asserted for a DP sink.

Signed-off-by: Sankeerth Billakanti <[email protected]>
Reviewed-by: Douglas Anderson <[email protected]>
Reviewed-by: Stephen Boyd <[email protected]>
Patchwork: https://patchwork.freedesktop.org/patch/483312/
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Dmitry Baryshkov <[email protected]>

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