drm/tests: mm: Convert to drm_dbg_printer

Fix one of the tests in drm_mm that was not converted prior to
drm_debug_printer removal, causing tests build failure.

Fixes: e154c4fc ("drm: remove drm_debug_printer in favor of drm_dbg_printer")
Signed-off-by: Michał Winiarski <michal.winiarski@intel.com>
Reviewed-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Reviewed-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240209140818.106685-1-michal.winiarski@intel.com