ENGR00290635-1 mxc vout:Fix a build warning

ENGR00290635-1 mxc vout:Fix a build warning

This patch fixes the following build warning by allocating
a block of virtual memory to cache an instance of the structure
mxc_vout_output instead of using the stack frame.
drivers/media/platform/mxc/output/mxc_vout.c: In function ‘mxc_vidioc_s_crop’:
drivers/media/platform/mxc/output/mxc_vout.c:1529:1: warning: the frame size of 1040 bytes is larger than 1024 bytes [-Wframe-larger-than=]

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