drm/i915: fix Haswell DP M/N registers

We have to write the correct values inside intel_dp_set_m_n and then
prevent these values from being overwritten later.

V2: Unconfuse double negation.

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>