drm/amdgpu: Keep the pflip interrupts always enabled v7

This fixes flickering issues caused by prematurely firing pflip
interrupts.

v2 (chk): add commit message, fix DCE V10/V11 and DM as well
v3: Re-enable pflip interrupt wherever we re-enable a CRTC
v4: Enable pflip interrupt in DAL as well
v5: drop DAL changes for upstream
v6: (agd): only enable interrupts on crtcs that exist
v7: (agd): integrate suggestions from Michel

Signed-off-by: Michel Dänzer <michel.daenzer@amd.com>
Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org