msm: mdss: add support for rotator downscale

Add support for rotator downscale, available from 8994. This allows
source data to be downscaled via power of 2 steps, before being
written back to memory.

Change-Id: If42a5a8c9cb336abfc22dc9848c24ec0f54cd878
Signed-off-by: Casey Piper <cpiper@codeaurora.org>
Signed-off-by: Saurabh Shah <saurshah@codeaurora.org>
Signed-off-by: Adrian Salido-Moreno <adrianm@codeaurora.org>