drm: Driver-specific ioctls range from 0x40 to 0x9f

DRM_COMMAND_END is 0xa0, so the last driver ioctl is 0x9f, not 0x99.

Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>