drm/i915: refactor panel backlight control functions. (v2)

This moves the functions into static const instead of having
funcs and data in the same struct.

It leaves the power callback alone, as it is used in a different
manner.

v2: leave power callback alone (Jani)

Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201130210945.31850-1-airlied@gmail.com