drm/i915: Fix stale comment for intel_ddi_pll_select()

Since the run-time PM on DPMS series, this function has an outdated
comment. Refresh it a bit.

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