drm/msm/dp: ensure dsc slice width does not exceed maximum width

A dp sink can support dsc slices up to a maximum
width specified by its capabilities register.
Ensure that the calculated dsc slice width does
not exceed this value.

Change-Id: I6b9a578ef5c763259f592d59638e1b7352064d7f
Signed-off-by: Fuad Hossain <fhossain@codeaurora.org>
Signed-off-by: Tatenda Chipeperekwa <tatendac@codeaurora.org>