drm/amdgpu: Use delayed work to collect RAS error counters

On Context Query2 IOCTL return the correctable and
uncorrectable errors in O(1) fashion, from cached
values, and schedule a delayed work function to
calculate and cache them for the next such IOCTL.

v2: Cancel pending delayed work at ras_fini().
v3: Remove conditionals when dealing with delayed
    work manipulation as they're inherently racy.

Cc: Alexander Deucher <Alexander.Deucher@amd.com>
Cc: Christian König <christian.koenig@amd.com>
Cc: John Clements <john.clements@amd.com>
Cc: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Luben Tuikov <luben.tuikov@amd.com>
Reviewed-by: Alexander Deucher <Alexander.Deucher@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>