drm/i915: Disable M2 frac division for integer case

v2 : Handle M2 frac division for both M2 frac and int cases

v3 : Addressed Ville's review comments. Cleared the old bits for RMW

v4 : Fix feedfwd gain (Ville)

Signed-off-by: Vijay Purushothaman <vijay.a.purushothaman@linux.intel.com>
Reviewed-by: Ville Syrjala <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>