msm: rotator: add downscale support in rotator

Rotator is capable of performing 1/2, 1/4 and 1/8 down scaling. Add
downscale_ratio parameter to provide support for this feature.

CRs-Fixed: 315874
Change-Id: I57dc3456ccc9bec2a125750b3e9909ffcbaf45f6
Signed-off-by: Adrian Salido-Moreno <adrianm@codeaurora.org>