MLK-11918-2 video: mxc ipuv3 fb: Return err in ->fb_check_var if fbi_tmp is NULL

MLK-11918-2 video: mxc ipuv3 fb: Return err in ->fb_check_var if fbi_tmp is NULL

It has to be successful to find a relevant background framebuffer for a
foreground framebuffer.  So, we should return error code if it happens
to fail.

This patch fixes the following issue reported by Coverity:
Dereference after null check (FORWARD_NULL)
var_deref_op: Dereferencing null pointer fbi_tmp.
		if (fbi_tmp->var.vmode & FB_VMODE_INTERLACED)
			var->vmode |= FB_VMODE_INTERLACED;
		else
			var->vmode &= ~FB_VMODE_INTERLACED;

Signed-off-by: Liu Ying <Ying.Liu@freescale.com>