staging: drm/omap: defer unpin until scanout completes

When flipping, defer unpinning until scanout completes, as indicated
by the appropriate END_WIN irq.

This also re-organizes things a bit, in replacing omap_fb_{pin,unpin}
with omap_fb_replace(), to make it easier to add support for scanout
synchronized DMM refill mode (flipping by just reprogramming DMM
synchronized with DSS scanout).

Signed-off-by: Rob Clark <rob@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>