drm/msm/dsi: Prevent signed BPG offsets from bleeding into adjacent bits

The bpg_offset array contains negative BPG offsets which fill the full 8
bits of a char thanks to two's complement: this however results in those
bits bleeding into the next field when the value is packed into DSC PPS
by the drm_dsc_helper function, which only expects range_bpg_offset to
contain 6-bit wide values.  As a consequence random slices appear
corrupted on-screen (tested on a Sony Tama Akatsuki device with sdm845).

Use AND operators to limit these two's complement values to 6 bits,
similar to the AMD and i915 drivers.

Fixes: b9080324 ("drm/msm/dsi: add support for dsc data")
Reviewed-by: Abhinav Kumar <quic_abhinavk@quicinc.com>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Signed-off-by: Marijn Suijten <marijn.suijten@somainline.org>
Patchwork: https://patchwork.freedesktop.org/patch/508941/
Link: https://lore.kernel.org/r/20221026182824.876933-11-marijn.suijten@somainline.org


Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>