OMAP2PLUS: DSS2: Cleanup clock source related code

Clean up some of the DSS functions which select/get clock sources, use switch
to select the clock source members since more clock sources will be introduced
later on.

Remove the use of macro CONFIG_OMAP2_DSS_DSI in dispc_fclk_rate, use a dummy
inline for function for dsi_get_pll_hsdiv_dispc_rate() instead for code clarity.

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