drm/msm/dpu: document INTF_EDP/INTF_DP difference

drm/msm/dpu: document INTF_EDP/INTF_DP difference

Based on the discussions on the mailing list, document enum
dpu_intf_type and it's controversial fields: INTF_DP and INTF_EDP.

INTF_EDP is used for older eDP interface found on msm8x74/msm8x84
INTF_DP is used for both eDP and DP interfaces handled by the msm/dp
driver. The DPU driver does not make a difference between them.

Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Reviewed-by: Stephen Boyd <swboyd@chromium.org>
Patchwork: https://patchwork.freedesktop.org/patch/475555/
Link: https://lore.kernel.org/r/20220222062246.242577-2-dmitry.baryshkov@linaro.org
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>