drm/panfrost: perfcnt: fix ref count leak in panfrost_perfcnt_enable_locked

in panfrost_perfcnt_enable_locked, pm_runtime_get_sync is called which
increments the counter even in case of failure, leading to incorrect
ref count. In case of failure, decrement the ref count before returning.

Acked-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Signed-off-by: Navid Emamdoost <navid.emamdoost@gmail.com>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200614063619.44944-1-navid.emamdoost@gmail.com