drm/vmwgfx: Support the NOP_ERROR command

Can be used by user-space applications to test and verify the kernel
command buffer error recovery functionality.

Malicious user-space apps could potentially use this command to slow down
graphics processing somewhat, but they could also accomplish the same thing
using a random malformed command so this should be considered safe.
At least as safe as it gets.

Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
Reviewed-by: Brian Paul <brianp@vmware.com>
Reviewed-by: Sinclair Yeh <syeh@vmware.com>