drm/i915/skl: Don't expose the top most plane on gen9 display

on SKL/BXT, the top most plane hardware is shared between the legacy
cursor registers and an actual plane. Daniel and Ville don't want to
expose 2 DRM planes and would rather expose a CURSOR plane that has all
the usual plane properties, and that's a blocker for lifting the
prelimary_hw_support flag.

Unfortunately noone has had the time to finish this yet, but lifting the
prelimary_hw_support flag is long overdue. As an intermediate solution
we can merely not expose the top most plane

Cc: Imre Deak <imre.deak@intel.com>
Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>