OMAP: DSS2: DISPC: Shorten _dispc_set_color_conv_coef()

Iterate over overlay id's to shorten _dispc_set_color_conv_coef()

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