drm/msm/dpu: add missing dpu_encoder kerneldoc

Describe missing dpu_encoder_phys_wb_is_master() argument and struct
dpu_encoder_virt.wide_bus_en field.

Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Reviewed-by: Abhinav Kumar <quic_abhinavk@quicinc.com>
Patchwork: https://patchwork.freedesktop.org/patch/518794/
Link: https://lore.kernel.org/r/20230118020549.1690078-1-dmitry.baryshkov@linaro.org


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