drm/omap: HDMI PLL: use runtime pm

To make it possible to use HDMI PLL for other video outputs than HDMI,
the HDMI PLL code needs to do runtime_get/put for the HDMI IP, so that
the IP (include the PLL) is enabled.

To do that we also need to store the HDMI pdev in the hdmi_pll_data.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>