vc4: Use GPIO interrupts to detect HDMI HPD state transitions.

[anholt]

Right now the DRM core is helping us out by polling the HPD periodically, but we should be able to just register for an interrupt, saving CPU wakeups.

[anholt]

This will get us the interrupts:

ret = devm_request_irq(dev,
                                 gpio_to_irq(hdmi->hpd_gpio),
                                 vc4_hdmi_hpd_irq_handler,
                                 IRQF_TRIGGER_RISING |
                                 IRQF_TRIGGER_FALLING,
                                 "HDMI HPD",
                                 vc4);

but we'd need a delayed work to go off and lock the mode config mutex so that we can do the poll.