drm/msm/dpu: add support for dither block in display

This change enables dither block for primary interface
in display.

Enabled for 6bpc in the current version.

Changes in v1:
 - Remove redundant error checks (Rob).

Signed-off-by: Kalyan Thota <kalyan_t@codeaurora.org>
Tested-by: Douglas Anderson <dianders@chromium.org>
Tested-by: Kristian H. Kristensen <hoegsberg@google.com>
Signed-off-by: Rob Clark <robdclark@chromium.org>