[stable][PATCH backport to 3.8.y.z extended stable] drm/i915: Make data/link N value power of two

[Haitao Zhang]

This back ported patch fixed the following bug:
Lenovo M4400s Screen will blink between normal and green
https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1212376


commit a65851af59387146a28a928c3e7bb17dabc5db72 in upstream

[PATCH] drm/i915: Make data/link N value power of two 

The BIOS uses power of two values for the data/link N value.

Follow suit to make the Zotac DP to dual-HDMI dongle work.

v2: Clean up the magic numbers and defines
    Change the N clamping to be a bit easier on the eye 
    Rename intel_reduce_ratio to intel_reduce_m_n_ratio

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=49402
Signed-off-by: Ville Syrjälä <ville.syrjala at linux.intel.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=59810
Tested-by:  Jesse Barnes <jbarnes at virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter at ffwll.ch>
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