drm/tests: mm: Call drm_mm_print in drm_test_mm_debug

The original intent behind the test was to sanity check whether calling
the debug iterator (drm_mm_print) doesn't cause any problems.
Unfortunately - this call got accidentally removed during KUnit
transition. Restore it.

Signed-off-by: Michał Winiarski <michal.winiarski@intel.com>
Signed-off-by: Maxime Ripard <mripard@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20240116174602.1019512-1-michal.winiarski@intel.com