drm/msm/dsi: Use DSC slice(s) packet size to compute word count

drm/msm/dsi: Use DSC slice(s) packet size to compute word count

According to downstream the value to use for WORD_COUNT is
bytes_per_pkt, which denotes the number of bytes in a packet based on
how many slices have been configured by the panel driver times the
width of a slice times the number of bytes per pixel.

The DSC panels seen thus far use one byte per pixel, only one slice
per packet, and a slice width of half the panel width leading to the
desired bytes_per_pkt+1 value to be equal to hdisplay/2+1.  This however
isn't the case anymore for panels that configure two slices per packet,
where the value should now be hdisplay+1.

Note that the aforementioned panel (on a Sony Xperia XZ3, sdm845) with
slice_count=1 has also been tested to successfully accept slice_count=2,
which would have shown corrupted output previously.

Fixes: 08802f515c3c ("drm/msm/dsi: Add support for DSC configuration")
Signed-off-by: Marijn Suijten <marijn.suijten@somainline.org>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Reviewed-by: Abhinav Kumar <quic_abhinavk@quicinc.com>
Patchwork: https://patchwork.freedesktop.org/patch/515694/
Link: https://lore.kernel.org/r/20221221231943.1961117-3-marijn.suijten@somainline.org
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>