drm/i915: Do not advertize support for NV12 on ICL yet.

ICL requires two planes for scanning out a NV12 framebuffer. Do
not advertize support for creating NV12 framebuffers until required
plane programming is implemented.

v2: Do not allow adding buffers.
    Check inside skl_plane_has_planar (Ville)

Bspec: Plane Planar YUV programming (18566)
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180824203856.17700-2-dhinakaran.pandiyan@intel.com