drm/i915: Pass down brightness values to enable/disable backlight callbacks

Instead of using intel_panel->backlight.level, have the caller provide us
with the current panel backlight value. We'll need this for when we
separate PWM-related backlight callbacks from other means of backlight
control (like DPCD backlight controls), as the caller of each PWM callback
will be responsible for converting the current brightness value to it's
respective PWM level.

Signed-off-by: Lyude Paul <lyude@redhat.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201204223603.249878-4-lyude@redhat.com