drm/msm/dpu: Only enable DSC_MODE_MULTIPLEX if dsc_merge is enabled

drm/msm/dpu: Only enable DSC_MODE_MULTIPLEX if dsc_merge is enabled

When the topology calls for two interfaces on the current fixed topology
of 2 DSC blocks, or uses 1 DSC block for a single interface (e.g. SC7280
with only one DSC block), there should be no merging of DSC output.

This is already represented by the return value of
dpu_encoder_use_dsc_merge(), but not yet used to correctly configure
this flag.

Fixes: 58dca9810749 ("drm/msm/disp/dpu1: Add support for DSC in encoder")
Signed-off-by: Marijn Suijten <marijn.suijten@somainline.org>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Patchwork: https://patchwork.freedesktop.org/patch/577067/
Link: https://lore.kernel.org/r/20240204-dpu-dsc-multiplex-v1-1-080963233c52@somainline.org
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>