OMAP: DSS2: OMAPFB: Remove implicit display update on unblank

Currently omapfb does an implicit display update (for manual update
displays) on unblank.

There is no guarantee that the framebuffer contains a valid image when
unblank is called. When using manual update displays it is the
responsibility of the user space to update the display, and so it should
be in this case also.

This patch removes the implicit display update on unblank.

Signed-off-by: Jani Nikula <ext-jani.1.nikula@nokia.com>
[tomi.valkeinen@ti.com: improved description]
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>