drm/panel: otm8009a: Fix glitches by moving backlight enable to otm8009a_enable()

The backlight 1st update was in the otm8009a_prepare() function for a
bad reason: backlight was not working in video mode and the
otm8009a_prepare() is in command mode for the init sequence. As the
backlight is now fixed (no low-power mode), it is good to put it back
in the otm8009a_enable() function, avoiding also image glitches visible
on some "slow" devices.

Signed-off-by: Philippe Cornu <philippe.cornu@st.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180423141054.13128-3-philippe.cornu@st.com