drm/vmwgfx: Switch to using DRM_IOCTL_DEF_DRV

The macro has been accounting for DRM_COMMAND_BASE for a long time
now so there's no reason to still be duplicating it. Plus we were
leaving the name undefined which meant that all the DRM ioctl
warnings/errors were always listing "null" ioctl at the culprit.

This fixes the undefined ioctl name and removes duplicated code.

Signed-off-by: Zack Rusin <zackr@vmware.com>
Reviewed-by: Martin Krastev <krastevm@vmware.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210723165153.113198-1-zackr@vmware.com