OMAPDSS: HDMI: split PLL enable & config

At the moment we have one function, hdmi_pll_enable, which enables the
PLL and writes the PLL configuration to registers.

To make the HDMI PLL ahere to the DSS PLL API, split the hdmi_pll_enable
into two parts: hdmi_pll_enable which enables the PLL HW, and
hdmi_pll_set_config which writes the config.

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