drm/bridge/synopsys: dsi: allow LP commands in video mode

Current code only sends LP commands in command mode.

Allows sending LP commands also in video mode by setting the
proper flag in DSI_VID_MODE_CFG.

Signed-off-by: Antonio Borneo <antonio.borneo@st.com>
Tested-by: Philippe Cornu <philippe.cornu@st.com>
Reviewed-by: Philippe Cornu <philippe.cornu@st.com>
Acked-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200708140836.32418-1-yannick.fertre@st.com