drm/msm: enumerate DSI interfaces

Follow the DP example and define MSM_DSI_CONTROLLER_n enumeration.

Reviewed-by: Abhinav Kumar <quic_abhinavk@quicinc.com>
Reviewed-by: Marijn Suijten <marijn.suijten@somainline.org>
Tested-by: Marijn Suijten <marijn.suijten@somainline.org>
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Patchwork: https://patchwork.freedesktop.org/patch/545351/
Link: https://lore.kernel.org/r/20230704022136.130522-2-dmitry.baryshkov@linaro.org