drm/msm/dsi: Remove incorrect references to slice_count

drm/msm/dsi: Remove incorrect references to slice_count

Currently, slice_count is being used to calculate word count and
pkt_per_line. Instead, these values should be calculated using slice per
packet, which is not the same as slice_count.

Slice count represents the number of slices per interface, and its value
will not always match that of slice per packet. For example, it is possible
to have cases where there are multiple slices per interface but the panel
specifies only one slice per packet.

Thus, use the default value of one slice per packet and remove slice_count
from the aforementioned calculations.

Fixes: 08802f515c3c ("drm/msm/dsi: Add support for DSC configuration")
Fixes: bc6b6ff8135c ("drm/msm/dsi: Use DSC slice(s) packet size to compute word count")
Reviewed-by: Marijn Suijten <marijn.suijten@somainline.org>
Signed-off-by: Jessica Zhang <quic_jesszhan@quicinc.com>
Patchwork: https://patchwork.freedesktop.org/patch/541965/
Link: https://lore.kernel.org/r/20230405-add-dsc-support-v6-5-95eab864d1b6@quicinc.com
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>