drm/msm/dsi: use RMW cycles in dsi_update_dsc_timing

drm/msm/dsi: use RMW cycles in dsi_update_dsc_timing

The downstream uses read-modify-write for updating command mode
compression registers. Let's follow this approach. This also fixes the
following warning:

drivers/gpu/drm/msm/dsi/dsi_host.c:918:23: warning: variable 'reg_ctrl' set but not used [-Wunused-but-set-variable]

Reported-by: kernel test robot <lkp@intel.com>
Fixes: 08802f515c3c ("drm/msm/dsi: Add support for DSC configuration")
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Reviewed-by: Abhinav Kumar <quic_abhinavk@quicinc.com>
Reviewed-by: Marijn Suijten <marijn.suijten@somainline.org>
Patchwork: https://patchwork.freedesktop.org/patch/484305/
Link: https://lore.kernel.org/r/20220430175533.3817792-1-dmitry.baryshkov@linaro.org
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>