OMAPDSS: HDMI: store WP pointer to hdmi_pll_data

HDMI PLL code needs the pointer to the WP block so that it can manage
its power. Currently this is passed as a function parameter to
hdmi_pll_enable and hdmi_pll_disable. To make the PLL function adhere to
the DSS PLL API, we need to remove the WP parameter.

This patch stores the WP pointer to hdmi_pll_data in hdmi_pll_init, so
that it's available when needed.

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