drm/radeon/kms: fix MSI re-arm on rv370+

MSI_REARM_EN register is a write only trigger register.
There is no need RMW when re-arming.

May fix:
https://bugs.freedesktop.org/show_bug.cgi?id=41668



Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
CC: stable@vger.kernel.org
Signed-off-by: Dave Airlie <airlied@redhat.com>