ENGR00253927 IPU: Fix NULL pointer bug when BG EOF interrupt occur early

ENGR00253927 IPU: Fix NULL pointer bug when BG EOF interrupt occur early

Fix NULL pointer bug when IPU BG EOF interrupt occur before register irq handler
It can be reproduced on MIPI DSI display on i.mx6dl SabreSD board.

The sequence is:
a) enable display channel
b) pan display (fb_set_var()) -> ipu_enable_irq
c) ipu_request_irq
If an EOF interrupt comes before c and after b, the issue happens.

Signed-off-by: Wayne Zou <b36644@freescale.com>