drm/radeon: fix UVD IRQ support on SI

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>
Cc: stable@vger.kernel.org
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>