drm/amdgpu: Reset IH OVERFLOW_CLEAR bit

Allows us to detect subsequent IH ring buffer overflows as well.

Cc: Joshua Ashton <joshua@froggi.es>
Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: Christian König <christian.koenig@amd.com>
Cc: stable@vger.kernel.org
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>