drm/msm/sde: don't clear topology name during test-only

Fix the test-only path so that a test-only operation does not
unintentionally clear the topology name property, by checking
if there is an existing reservation before clearing the topology
name.

CRs-Fixed: 2005394
Change-Id: I40253359354d846c797b27f26a7ea7a77582d461
Signed-off-by: Lloyd Atkinson <latkinso@codeaurora.org>