I've done some extensive history digging across libdrm, mesa and
xf86-video-{intel,nouveau,ati}. The only potential user of this with
kms drivers I could find was ttmtest, which once used drmGetLock
still. But that mistake was quickly fixed up. Even the intel xvmc
library (which otherwise was really good with using dri1 stuff in kms
mode) managed to never take the hw lock for dri2 (and hence kms).

Hence it should be save to unconditionally disallow this.

Cc: Peter Antoine <peter.antoine@intel.com>
Reviewed-by: Peter Antoine <peter.antoine@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>