drm/panfrost: Disable PM on probe failure

Make sure to disable runtime PM again if probe fails after we've enabled
it. Otherwise, any subsequent attempt to re-probe starts triggering
"Unbalanced pm_runtime_enable!" assertions from the driver core.

Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/2487391e7646cabbc52e9b4c20182e39d3f61859.1556195258.git.robin.murphy@arm.com