The method for change between scaling and subsampling mode is different
from ov5640_mipi.
(image bigger than 1280*960 is scaling mode, smaller is subsampling).

According to OV5640 Auto Focus Camera Module Application Notes
(with DVP Interface) R2.14.pdf,
change back from QSXGA to VGA, don't need to do exposure calculation.

According to the test result, if we do exposure calculation when change
back from scaling to subsampling mode, the image would be dark.

So the method is:
Change to scaling mode, go through exposure calcuation.
Change to or back to subsampling mode, change mode directly.

Supported mode:
- QSXGA@7.5fps
- 1080P@7.5fps

Can't make 1080P works at 15fps. Here is a reply from ov fae:
because of scaling down, max frame for 1080P is the same as 5M, both are 15fps.
so if 5M can runs up to 7.5fps on your demo, then 1080P is the same 7.5fps max.

Signed-off-by: Sheng Nan <b38800@freescale.com>