drm/omap: Use old_fb to synchronize between successive page flips

omap_crtc->old_fb is used to check whether the previous page flip has completed
or not. However, it's never initialized to anything, so it's always NULL. This
results in the check to always succeed, and the page_flip to proceed.

Initialize old_fb to the fb that we intend to flip to through page_flip, and
therefore prevent a future page flip to proceed if the last one didn't
complete.

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