drm/i915: Fix TV get_modes to return modes count

The get_modes hook must return the number of modes added.  This also fixes
TV mode's clock calculation int overflow issue, and use 0.01 precision for
mode refresh validation.

Signed-off-by: Zhenyu Wang <zhenyu.z.wang@intel.com>
Signed-off-by: Eric Anholt <eric@anholt.net>