drm: Fix DRM_IOCTL_DEF_DRV()

Currently DRM_IOCTL_DEF_DRV does '[DRM_IOCTL_NR(DRM_##ioctl)]' which
doesn't make much sense since DRM_##ioctl is already a the raw ioctl
number. So change it to 'DRM_IOCTL_NR(DRM_IOCTL_##ioctl) - DRM_COMMAND_BASE'
which means the DRM_IOCTL_NR() now makes sense, and also this also means
if there's a mistake in the DRM_IOCTL_##ioctl macros we might get a
warning about it (eg. we would have gotten a sparse warning about the
i915 colorkey get/set ioctl being defined to be the same thing).

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>