drm/vmwgfx: Replace unconditional mutex unlocked warnings with lockdep counterpart

Replace instances of WARN_ON[_ONCE](!mutex_is_held()) with
lockdep_assert_held(). This makes sure the checking process actually
holds the mutex and also removes the checks from release builds

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