drm/nouveau: fix reference count leak in nouveau_debugfs_strap_peek

[ Upstream commit 8f294324 ]

nouveau_debugfs_strap_peek() calls pm_runtime_get_sync() that
increments the reference count. In case of failure, decrement the
ref count before returning the error.

Signed-off-by: Aditya Pakki <pakki001@umn.edu>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>