drm/radeon/pm: don't allow debugfs/sysfs access when PX card is off (v2)

When the PX card is off don't try and access it. Avoid hw access
to the card while it's off (e.g., reading back invalid temperature).

v2: be less strict

bug:
https://bugzilla.kernel.org/show_bug.cgi?id=76321

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org
Signed-off-by: Christian König <christian.koenig@amd.com>