drm/i915/skl: Don't create a VGA connector on Skylake

v2: Rebase on top of the intel_crt_present() addition
v3: Fix rebase error (we were patching the wrong function)

Reviewed-by: Thomas Wood <thomas.wood@intel.com>
Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>