Equivalent change to the radeon driver.

Note that with radeon this caught a bug in the dri3 DDX
implementation, which asked for vblank interrupts when the pipe is
off. That bug needs to be fixed before we can merge this patch (if
amdgpu is affected too). Michel discovered this one.

v2: agd: switch dce8 as well.
Acked-by: Michel Dänzer <michel.daenzer@amd.com>
Cc: Michel Dänzer <michel.daenzer@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>