drm/radeon: add a GPU reset counter queryable by userspace

Userspace will be able to tell whether a GPU reset occured by comparing
an old referece value of the counter with a new value.

Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Marek Olšák <marek.olsak@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>