drm/i915/panel: Add missing parameters to kerneldoc

drivers/gpu/drm/i915/intel_panel.c:409: warning: Function parameter or member 'source_min' not described in 'scale'
drivers/gpu/drm/i915/intel_panel.c:409: warning: Function parameter or member 'source_max' not described in 'scale'
drivers/gpu/drm/i915/intel_panel.c:409: warning: Function parameter or member 'target_min' not described in 'scale'
drivers/gpu/drm/i915/intel_panel.c:409: warning: Function parameter or member 'target_max' not described in 'scale'

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180214091747.12753-1-chris@chris-wilson.co.uk
Link: https://patchwork.freedesktop.org/patch/msgid/20180214091747.12753-2-chris@chris-wilson.co.uk