drm/vmwgfx: Make buffer object lookups reference-free during validation

Make the process of looking up a buffer object and adding it to the
validation list reference-free unless when it's actually added to the
validation list where a single reference is taken.
This saves two locked atomic operations per command stream buffer object
handle lookup.

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