drm/i915: Document the find_pll() function

The proper use of find_pll() isn't always so easy to determine from the
code itself. Some documentation should help.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>