drm/amdgpu/gfx8: fix priv reg interrupt enable

Looks like a copy/paste typo.

Reviewed-by: Christian König <christian.koenig@amd.com>
Noticed-by: David Panariti <David.Panariti@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>