drm/i915/backlight: drop drm_device local variables in favor of i915

Prefer only having struct drm_i915_private *i915 around. Drop the
drm_device *dev locals.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/6a791b06ab84bb8fb719cd46934eb09644e3edc7.1670405587.git.jani.nikula@intel.com