drm/i915: s/ironlake_/intel_ for the enable_share_dpll function

Besides the fairly useless BUG_ON the logic is completely generic
and cane be used on any platform what wants to reuse the shared
dpll support code.

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