drm: drm_ut_debug_printk() isn't called with NULL anywmore

The DRM_LOG* macros where the only sites where drm_ut_debug_printk was
called with NULL arguments for prefix and function_name. Now that they
are gone, we can remove that case.

Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>