drm/i915: move backlight level setting in enable/disable to hooks

This allows more flexibility in the ordering of the register writes, and
lets us drop level setting altogether as necessary on a per platform
basis.

For gen2-gen3, this is the only thing that happens in enable/disable.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>