drm: reorder struct drm_ioctl_desc to save space on 64 bit builds

shrinks drm_ioctl_desc from 24 bytes to 16 bytes by reordering members
to remove padding.

updates DRM_IOCTL_DEF macro to initialise structure members by name to
handle the structure reorder.

The applied patch reduces data used in drm.ko from 10440 to 9032

Signed-off-by: Richard Kennedy <richard@rsk.demon.co.uk>
Signed-off-by: Dave Airlie <airlied@redhat.com>