drm/i915: keep max backlight internal to intel_panel.c

In preparation of adding locking to backlight, make max backlight value
(the modulation frequency the PWM duty cycle value must not exceed)
internal to intel_panel.c.

Have intel_panel_set_backlight() accept a caller defined range for level,
and scale input to max backlight value internally.

Clean up intel_panel_get_max_backlight() and usage internally.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>