drm/i915: panel: Use intel_panel_compute_brightness() from pwm_setup_backlight()

Use intel_panel_compute_brightness() from pwm_setup_backlight() so that
we correctly take i915_modparams.invert_brightness and/or
QUIRK_INVERT_BRIGHTNESS into account when setting + getting the initial
brightness value.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200221172927.510027-1-hdegoede@redhat.com