drm/amdgpu: Reset IH OVERFLOW_EN bit for IH 7.0

IH 7.0 support landed shortly after the original patch for resetting the
bit on all other generations, but without that patch applied.

Fixes: 12443fc5 ("drm/amdgpu: Add ih v7_0 ip block support")
Cc: Christian König <christian.koenig@amd.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Friedrich Vock <friedrich.vock@gmx.de>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>