drm/radeon: fix oops in ttm reserve when pageflipping (v2)

We need to take a reference to this object, pinning doesn't take a reference
so if userspace deletes the object it can disappear even if pinned.

v2: fix error paths to unreference properly also.

should fix:
https://bugzilla.kernel.org/show_bug.cgi?id=32402
and
https://bugzilla.redhat.com/show_bug.cgi?id=680651

Acked-By: Alex Deucher <alexdeucher@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>