drm/i915: Allow fuzzy matching in intel_compare_link_m_n

This prevents a unnecessary modeset on a dell XPS 13 (2016).

N is always a power of 2, which means that for fuzzy matching we should
compare for inequality on the n values, then do fuzzy matching on the m
values.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Tested-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/568D0E93.304@linux.intel.com