MLK-11536 video: mxc ipuv3 fb: Fix pan-disp frame drop issue when PRE is enabled

MLK-11536 video: mxc ipuv3 fb: Fix pan-disp frame drop issue when PRE is enabled

There could be frame drop issue when we do pan display if we update PRE next
buffer address before waiting for the flip completion, because we may hold
the flip completion already and then two continuous pan display operations may
happen quickly within one active period of frame scanning - the first PRE next
buffer address is overriden.  To fix this issue, this patch updates the buffer
address after the flip completion.

Signed-off-by: Liu Ying <Ying.Liu@freescale.com>
(cherry picked from commit d0126e42f46f6dad9ae8c6aa390b0e1bea76492e)