drm/radeon/vm: don't attempt to update ptes if ib allocation fails

commit 4cc948b94a222c310ae089c36718aac7a03aec90 upstream.

If we fail to allocate an indirect buffer (ib) when updating
the ptes, return an error instead of trying to use the ib.
Avoids a null pointer dereference.

Bug:
https://bugzilla.kernel.org/show_bug.cgi?id=58621



v2 (chk): rebased on drm-fixes-3.12 for stable inclusion

Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>