drm/i915/uncore: split unclaimed_reg_debug() to header and footer

Make it easier to have different logic for the two for follow-up.

Cc: Lee Shawn C <shawn.c.lee@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/8a0a93f08314f8d7e222a907d9aa5e0b89cb969e.1690886109.git.jani.nikula@intel.com