drm/i915/skl: Restore pipe B/C interrupts

Extending BDW implementation to gen9. Pipe B/C interrupt
restoration after exiting LPSP.

v2: Fix minor rebasing conflict.

Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Satheeshakrishna M <satheeshakrishna.m@intel.com>
Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>