My 855gm doesn't register the intel backlight but it still ends up
calling the backlight code to enable/disable the backlight via the
LVDS code. This leads to some WARNs due to backlight.max being 0.

Let's have intel_panel_enable_backlight() and intel_panel_disable_backlight()
check whether there's a backlight present or not.

Also move the backlight.present check from asle_set_backlight() into
intel_panel_set_backlight() for some extra symmetry.
Signed-off-by: NVille Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: NJani Nikula <jani.nikula@intel.com>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>