OMAP: DSS2: remove non-working msleep(40) workaround

The sleep workaround for the problem described in the comment doesn't
really work. This patch removes the workaround, and improves the comment
about the bug, and proposes an userspace workaround.

omap_dss_set_manager() is called 6 times when loading omapdss and
omapfb, which means that 40ms * 6 = 240ms was spent sleeping when
booting up.

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