drm/i915/bdw: Broadwell has 3 pipes

v2: Rebase (Paulo Zanoni)

v3: Rebase on top of num_pipes having moved to intel_device_info.

Signed-off-by: Damien Lespiau <damien.lespiau@intel.com> (v1)
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com> (v2)
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>