drm/i915 fixes for v5.6-rc3:

- Workaround missing Display Stream Compression (DSC) state readout by
  forcing modeset when its enabled at probe
- Fix EHL port clock voltage level requirements
- Fix queuing retire workers on the virtual engine
- Fix use of partially initialized waiters
- Stop using drm_pci_alloc/drm_pci/free
- Fix rewind of RING_TAIL by forcing a context reload
- Fix locking on resetting ring->head
- Propagate our bug filing URL change to stable kernels