OMAPDSS: clean up dss_mgr_set_lcd_config

dss_mgr_set_lcd_config() can only be called when the output is not
active. This means that most of the code in the function is extra, as
there's no need to write the values to registers, etc, because that will
be handled when the output will be enabled.

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