drm/vmwgfx: Adapt execbuf to the new validation api

Strip the old execbuf validation functionality and use the new API instead.
Also use the new API for a now removed execbuf function that was called
from the kms code.

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