vmwgfx: Fix unitialized stack read in vmw_setup_otable_base

One of the error paths in vmw_setup_otable_base causes us to return with
'ret' having never been set to anything causing us to return whatever was
on the stack.

Found with Coverity

Signed-off-by: Dave Jones <davej@fedoraproject.org>
Reviewed-by: Thomas Hellstrom <thellstrom@vmware.com>