drm/i915: fix up ivb plane 3 pageflips

Or at least plug another gapping hole. Apparrently hw desingers only
moved the bit field, but did not bother ot re-enumerate the planes
when adding support for a 3rd pipe.

Discovered by i-g-t/flip_test.

This may or may not fix the reference bugzilla, because that one
smells like we have still larger fish to fry.

v2: Fixup the impossible case to catch programming errors, noticed by
Chris Wilson.

References: https://bugs.freedesktop.org/show_bug.cgi?id=50069
Acked-by: Chris Wilson <chris@chris-wilson.co.uk>
Tested-by: Eugeni Dodonov <eugeni.dodonov@intel.com>
Eugeni Dodonov <eugeni.dodonov@intel.com>
Cc: stable@vger.kernel.org
Signed-Off-by: Daniel Vetter <daniel.vetter@ffwll.ch>