OMAPDSS: APPLY: add dss_set_go_bits()

Currently dss_write_regs() implicitely sets the GO bits for all managers
with shadow dirty flags set. This is a bit misleading, as one does not
presume "write registers" function to also set the GO bit.

Thus this patch splits the setting of GO bits into a separate function,
dss_set_go_bits, which is used after writing the registers.

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