drm/radeon/cik: switch to type3 nop packet for compute rings (v2)

Type 2 packets are deprecated on CIK MEC and we should use
type 3 nop packets.  Setting the count field to the max value
(0x3fff) indicates that only one dword should be skipped
like a type 2 packet.

v2: add comment to code

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Jerome Glisse <jglisse@redhat.com>