drm/radeon: fix multiple reference count leak

On calling pm_runtime_get_sync() the reference count of the device
is incremented. In case of failure, decrement the
reference count before returning the error.

Signed-off-by: Aditya Pakki <pakki001@umn.edu>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>