drm/panel: otm8009a: remove hack to force commands in HS

The panel is able to receive commands in LP. The current hack to
force backlight commands in HS was due to workaround an incorrect
settings on DSI controller that prevents sending LP commands while
video out was active.

Remove the hack that forces HS commands.

Changes in v2:
  - Added my signed-off

Signed-off-by: Antonio Borneo <antonio.borneo@st.com>
Signed-off-by: Yannick Fertre <yannick.fertre@st.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200922074242.28719-1-yannick.fertre@st.com