drm/radeon: fix UVD IRQ support on SI

commit b927e1c20462c1ad9caf4c4fa3a30e838a2d4037 upstream.

Otherwise decoding isn't really useable.

bug:
https://bugs.freedesktop.org/show_bug.cgi?id=71448



Signed-off-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>