drm: bridge: dw-hdmi: Extract PHY interrupt setup to a function

In preparation for adding PHY operations to handle RX SENSE and HPD,
group all the PHY interrupt setup code in a single location and extract
it to a separate function.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Reviewed-by: Jose Abreu <joabreu@synopsys.com>
[narmstrong: renamed dw_hdmi_fb_registered to dw_hdmi_setup_i2c]
Reviewed-by: Archit Taneja <architt@codeaurora.org>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>