drm/i915/skl: Restore pipe interrupt registers after power well enabling

The pipe interrupt registers are in the actual pipe power well, so we
need to restore them when re-enable the corresponding power well.

I've also copied what we do on HSW/BDW for VGA, even if the we haven't
enabled unclaimed registers just yet.

v2: Don't run skl_power_well_post_enable() if the power well is already
    enabled (Paulo)

Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>