drm/i915/skl: Implement the skl version of MMIO flips

Because the plane registers are different in Skylake we need to adapt
the MMIO code as well.

v2: Don't introduce yet another vfunc when the direction is do
consolidate the plane updates to use the same code path (Daniel)

v3:
  - Use enum pipe instead of int (Ville)
  - Also update PLANE_STRIDE when the tiling has changed (Ville)
  - Put intel_mark_page_flip_active() in the shared code (Damien)

v4:
  - Remove unused variable

v5:
  - Fix whitespace Vs tabs (Ville)

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>