drm/i915: support 3 pipes on IVB+

Well almost anyway.  IVB has 3 planes, pipes, transcoders, and FDI
interfaces, but only 2 pipe PLLs.  So two of the pipes must use the same
pipe timings (e.g. 2 DP plus one other, or two HDMI with the same mode
and one other, etc.).

Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Tested-By: Eugeni Dodonov <eugeni.dodonov@intel.com>
Reviewed-By: Eugeni Dodonov <eugeni.dodonov@intel.com>
Signed-off-by: Keith Packard <keithp@keithp.com>