drm/amdgpu: Return error if no RAS

In amdgpu_ras_query_error_count() return an error
if the device doesn't support RAS. This prevents
that function from having to always set the values
of the integer pointers (if set), and thus
prevents function side effects--always to have to
set values of integers if integer pointers set,
regardless of whether RAS is supported or
not--with this change this side effect is
mitigated.

Also, if no pointers are set, don't count, since
we've no way of reporting the counts.

Also, give this function a kernel-doc.

Cc: Alexander Deucher <Alexander.Deucher@amd.com>
Cc: John Clements <john.clements@amd.com>
Cc: Hawking Zhang <Hawking.Zhang@amd.com>
Reported-by: Tom Rix <trix@redhat.com>
Fixes: a46751fb

 ("drm/amdgpu: Fix RAS function interface")
Signed-off-by: Luben Tuikov <luben.tuikov@amd.com>
Reviewed-by: Alexander Deucher <Alexander.Deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>