drm/amdgpu: move non-DC vblank handling out of irq code

Move it into the DCE code for each generation. This avoids
confusion with the different display paths.

v2: no need for a hotplug worker for vkms

Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>