drm/radeon: only enable kv/kb dpm interrupts once v3

Enable at init and disable on fini. Workaround for hardware problems.

v2 (chk): extend commit message
v3: add new function

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Christian König <christian.koenig@amd.com> (v2)
Cc: stable@vger.kernel.org