drm/omap: clean up the LCD clk mux code

The code to set the clock muxes for DISPC's LCD clock inputs is very
confusing. Especially on DRA7, there's an additional clock muxing that
needs to be done, which at the moment is done in dpi.c using
dss_ctrl_pll_set_control_mux().

Clean this all up by:
- Using dss_clk_source instead of dss_pll_id, as dss_pll_id doesn't
  specify the clock source quite correctly.
- Splitting the dss_select_lcd_clk_source() up into DSS version specific
  helper functions.
- Using dss_ctrl_pll_set_control_mux() from the helper functions, so
  that dpi.c doesn't have to call it.

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