drm/msm/mdp5: use irqdomains

For mdp5, the irqs of hdmi/eDP/dsi0/dsi1 blocks get routed through the
mdp block.  In order to decouple hdmi/eDP/etc, register an irq domain
in mdp5.  When hdmi/dsi/etc are used with mdp4, they can directly setup
their irqs in their DT nodes as normal.  When used with mdp5, instead
set the mdp device as the interrupt-parent, as in:

	mdp: qcom,mdss_mdp@fd900000 {
		compatible = "qcom,mdss_mdp";
		interrupt-controller;
		#interrupt-cells = <1>;
		...
	};

	hdmi: qcom,hdmi_tx@fd922100 {
		compatible = "qcom,hdmi-tx-8074";
		interrupt-parent = <&mdp>;
		interrupts = <8 0>;   /* MDP5_HW_INTR_STATUS.INTR_HDMI */
		...
	};

There is a slight awkwardness, in that we cannot disable child irqs
at the mdp level, they can only be cleared in the child block.  So
you must not use threaded irq handlers in the child.  I'm not sure
if there is a better way to deal with that.

Signed-off-by: Rob Clark <robdclark@gmail.com>