mdss: msm: Update dither driver interface

Post processing driver clients can program the dither table in mdp.
If length of the dither table is set to 0, driver will program the
default table. If driver client would like to program the table it needs
to update the length field as per mdp hardware version.

CRs-fixed: 983164
Change-Id: I5e6aaa3d9376884e5ea1fe153cdf2798e3a52d1e
Signed-off-by: Gopikrishnaiah Anandan <agopik@codeaurora.org>