drm/vc4: Respect GPIO_ACTIVE_LOW on HDMI HPD if set in the devicetree.

The original Raspberry Pi had the GPIO active high, but the later
models are active low.  The DT GPIO bindings allow specifying the
active flag, except that it doesn't get propagated to the gpiodesc, so
you have to handle it yourself.

Signed-off-by: Eric Anholt <eric@anholt.net>
Tested-by: Daniel Stone <daniels@collabora.com>