drm/imx: dw_hdmi-imx: use imx_drm_encoder_parse_of

This is the same code and comment that is already shared by imx-ldb,
imx-tve, and parallel-display in imx_drm_encoder_parse_of().

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>