drm/i915: reuse WRPLL when possible

It seems we do have machines with 3 HDMI/DVI outputs, so sharing
WRPLLs is the only way to get 3 pipes working.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=68485


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