drm/backlight: initialize struct backlight_properties properly

The power field was never correctly initialized.

[airlied: just took the two drm specific bits]

Signed-off-by: Corentin Chary <corentin.chary@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>

diff --cc drivers/gpu/drm/i915/intel_panel.c
Simple merge

diff --cc drivers/gpu/drm/radeon/radeon_legacy_encoders.c
Simple merge