drm: bridge/dw_hdmi: improve HDMI enable/disable handling

drm: bridge/dw_hdmi: improve HDMI enable/disable handling

HDMI sinks are permitted to de-assert and re-assert the HPD signal to
indicate that their EDID has been updated, which may not involve a
change of video information.

An example of where such a situation can arise is when an AV receiver
is connected between the source and the display device.  Events which
can cause the HPD to be deasserted include:

 * turning on or switching to standby the AV receiver.
 * turning on or switching to standby the display device.

Each of these can change the entire EDID data, or just a part of the
EDID data - it's up to the connected HDMI sink to do what they desire
here.  For example

 - with the AV receiver and display device both in standby, a source
   connected to the AV receiver may provide its own EDID to the source.
 - turning on the display device causes the display device's EDID to be
   made available in an unmodified form to the source.
 - subsequently turning on the AV receiver then provides a modified
   version of the display device's EDID.

Moreover, HPD doesn't tell us whether something is actually listening
on the HDMI TDMS signals.  The phy gives us a set of RXSENSE indications
which tell us whether there is a sink connected to the TMDS signals.

Currently, we use the HPD signal to enable or disable the HDMI block,
which is questionable when HPD is used in this manner.  Using the
RXSENSE would be more appropriate, but there is some bad behaviour
which needs to be coped with.  The iMX6 implementation lets the TMDS
signals float when the phy is "powered down", which cause spurious
interrupts.  Rather than just using RXSENSE, use RXSENSE and HPD
becoming both active to signal the presence of a device, but loss
of RXSENSE to indicate that the device has been unplugged.

The side effect of this change is that a sink deasserting the HPD
signal to cause a re-read of the EDID data will not cause the bridge
to immediately disable the video signal.

Tested-by: Philipp Zabel <p.zabel@pengutronix.de>
Reviewed-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>