drm/msm: add irq mapping for cec in hdmi driver

Make HDMI device as the interrupt parent of CEC device, so add
the new IRQ domain in HDMI driver dealing with the necessary
IRQ mapping.

Change-Id: Id935da1d1e488ccee01b831b9f085a83d67268f2
Signed-off-by: Ray Zhang <rayz@codeaurora.org>