drm/msm/dp: correct dsc params for dual-dsc config

Adjust the bytes-per-chunk configuration value for
dual-dsc merge use-case and add support for slice_height
of 15 lines for modes with height evenly divisable by 15.

CRs-Fixed: 2325207
Change-Id: I1863709f0c844c2810a877aa61ca9ab89e4cdeae
Signed-off-by: Govinda Rajulu Chenna <gchenna@codeaurora.org>