msm: mdss: enable dithering for panels with reduced color depth

In order to avoid banding caused by using reduced color depth as panel
output format, dither can be enabled to produce an image which gives
better representation of original image.

Change-Id: I88a1be747332070d2b8b00dcbeb7e97936a47092
Signed-off-by: Adrian Salido-Moreno <adrianm@codeaurora.org>