drm: Remove the prefix argument of drm_ut_debug_printk()

This is always DRM_NAME, so we can just make it part of the format
string instead of asking prink to do it for us.

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