OMAPDSS: HDMI: PLL changes for OMAP5

Add a features struct to differentiate between the HDMI PLLs on OMAP4
and OMAP5. The OMAP5 PLL is more sensitive when it comes to locking.  We
need to ensure that the DCO freq isn't too low for lower pixel clocks.

Modify the PLL computation slightly to ensure the HDMI PLL locks for lower
frequencies. This will be later replaced by a more complex computation
which makes sure all the PLL constraints are met.

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