drm: use drms ioctl cmd not what we get passed from userspace.

This enforces us to use the drm ioctl types so read/write works correctly and not believe
what userspace tells us.

It does this hopefully without breaking the drm api.

Fixes bug from thread: BUG: unable to handle kernel NULL pointer dereference (drm_getunique)

Signed-off-by: Dave Airlie <airlied@redhat.com>