drm/vc4: Fall back to using an EDID probe in the absence of a GPIO.

On Pi0/1/2, we use an external GPIO line for hotplug detection, since
the HDMI_HOTPLUG register isn't connected to anything.  However, with
the Pi3 the HPD GPIO line has moved off to a GPIO expander that will
be tricky to get to (the firmware is constantly polling the expander
using i2c0, so we'll need to coordinate with it).

As a stop-gap, if we don't have a GPIO line, use an EDID probe to
detect connection.  Fixes HDMI display on the pi3.

Signed-off-by: Eric Anholt <eric@anholt.net>