drm/amdgpu: fix kcq mqd_backup buffer double free for multi-XCD

For gfx_v9_4_3 and beyond, struct kiq has its own mqd_backup pointer
rather than using the last pointer from mec struct. Then the kfree
operation on the pointer from the mec struct should be removed otherwise
it will cause double free on the first kcq's mqd_backup buffer on XCD1.

Signed-off-by: Shiwu Zhang <shiwu.zhang@amd.com>
Reviewed-by: Le Ma <le.ma@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>