drm: Implement drmCheckModesettingSupported() for DragonFly

For the sake of simplicity, KMS support can always be considered
present on DragonFly.

If some particular version doesn't support KMS yet, appropriate
checks are already done in Dports's x11-drivers/ Makefiles and
KMS-enabled driver packages don't get built.

Signed-off-by: François Tigeot <ftigeot@wolfpond.org>
Signed-off-by: Maarten Lankhorst <dev@mblankhorst.nl>