drm/i915: move is_dual_link_lvds to intel_lvds.c

Just a prep patch to make this a property of intel_lvds. Makes more
sense, removes clutter from intel_display.c and eventually I want to
move all the encoder special cases wrt clock handling to encoders
anyway.

v2: Add an intel_ prefixe to is_dual_link_lvds since it's non-static
now.

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