drm/dp: Don't read back backlight mode in drm_edp_backlight_enable()

drm/dp: Don't read back backlight mode in drm_edp_backlight_enable()

As it turns out, apparently some machines will actually leave additional
backlight functionality like dynamic backlight control on before the OS
loads. Currently we don't take care to disable unsupported features when
writing back the backlight mode, which can lead to some rather strange
looking behavior when adjusting the backlight.

So, let's fix this by just not reading back the current backlight mode on
initial enable. I don't think there should really be any downsides to this,
and this will ensure we don't leave any unsupported functionality enabled.

This should fix at least one (but not all) of the issues seen with DPCD
backlight support on fi-bdw-samus

v5:
* Just avoid reading back DPCD register - Doug Anderson

Signed-off-by: Lyude Paul <lyude@redhat.com>
Fixes: 867cf9cd73c3 ("drm/dp: Extract i915's eDP backlight code into DRM helpers")
Reviewed-by: Douglas Anderson <dianders@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20211105183342.130810-4-lyude@redhat.com