drm/msm: fix DSC slice last_group_size config

Currently the DSC slice last_group_size configures
wrong values when the dsc slice is not a perfect
multiple of 3. Fix it to configure the correct value
for any dsc slice.

Change-Id: Ie2e83cfd801aa63f643b371f69b45bf78781ff74
Signed-off-by: Veera Sundaram Sankaran <veeras@codeaurora.org>