drm/i2c: tda998x: re-implement "Fix EDID read timeout on HDMI connect"

drm/i2c: tda998x: re-implement "Fix EDID read timeout on HDMI connect"

Commit a703b37ff359 ("drm: tda998x: Fix EDID read timeout on HDMI
connect") used a weak scheme to try and delay reading EDID on a HDMI
connect event.  It is weak because delaying the notification of a
hotplug event does not stop userspace from trying to read the EDID
within the 100ms delay.

The solution provided here solves this issue:
* When a HDMI connection event is detected, mark a blocking flag for
  EDID reads, and start a timer for the delay.
* If an EDID read is attempted, and the blocking flag is set, wait
  for the blocking flag to clear.
* When the timer expires, clear the blocking flag and wake any thread
  waiting for the EDID read.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>