drm/i915: Fix and cleanup DPLL calculation for Ironlake

When the ideal error range can't be reached, this will safely use
a most closed one. Clean up some dumb codes in DPLL function too.

This fixes DPLL clock issue against one monitor at 1680x1050@60hz.

Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Signed-off-by: Eric Anholt <eric@anholt.net>