drm/amdgpu: Fix KFD oversubscription by tracking queues correctly

The number of compute queues available to the KFD was erroneously
calculated as 64. Only the first MEC can execute compute queues and
it has 32 queue slots.

This caused the oversubscription limit to be calculated incorrectly,
leading to a missing chained runlist command at the end of an
oversubscribed runlist.

v2: Remove unused num_mec field to avoid duplicate logic
v3: Separate num_mec removal into separate patches

Change-Id: I9e7bba2cc1928b624e3eeb1edb06fdb602e5294f
Signed-off-by: Jay Cornwall <Jay.Cornwall@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Oded Gabbay <oded.gabbay@gmail.com>