drm/amdgpu: disable MMHUB PG for Picasso

commit f626dd0f

 upstream.

MMHUB PG needs to be disabled for Picasso for stability reasons.

Signed-off-by: Evan Quan <evan.quan@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>