drm: Remove the unused (and unusable) DRM_LOG_MODE()

This macro was trying to use the non existing DRM_UT_MODE debug category
and looks like it should be covered by DRM_LOG_KMS().

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