drm/i915/backlight: convert DRM_DEBUG_KMS() to drm_dbg_kms()

Fix the final straggler.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/4769f8377be11536bd19840a2e59ef9f8c0a558c.1670405587.git.jani.nikula@intel.com