drm/i915: Explicitly cast pipe -> intel_dpll_id

We only do this on IBX where there's a fixed pch dpll to pipe
assignment. Being explicit about it can't really hurt and makes
sparse happy.

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