drm/i915: Kill crtc->plane checks from the primary plane update hooks

These were apparently meant to protect the SAREA which only has
room for two pipes, but things clearly went a bit wonky when
first the .update_plane() hooks were split up and then pipe C
got introduced.

The checks actually protecting the SAREA live in
intel_crtc_update_sarea() these days, so the checks in the primary
plane update hooks are just historical leftovers which are to be
eliminated.

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