drm/radeon: fix active_cu mask on SI and CIK after re-init (v3)

Need to initialize the mask to 0 on init, otherwise it
keeps increasing.

bug:
https://bugzilla.kernel.org/show_bug.cgi?id=82581



v2: also fix cu count
v3: split count fix into separate patch

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>
Cc: stable@vger.kernel.org