drm/i915/uncore: fix race around i915->params.mmio_debug

Only check the conditions for unclaimed reg debug once to avoid locking
problems when i915->params.mmio_debug changes between header and footer.

Closes: https://gitlab.freedesktop.org/drm/intel/-/issues/8749
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/a53fb0fd84c4627398ccd4304b35db05603b89b6.1690886109.git.jani.nikula@intel.com