OMAP: DSS2: improve DSS clk src selection

dss_select_clk_source() was rather confusing. Selecting the source with
enums is much clearer.

The clk source selection is also stored into memory, so that we know what
is the selected source, even when clocks are off. This is important during
setup, as we need to what clocks to turn on before the clocks are turned
on.

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