drm/i915: remove comment about IVB link training from intel_pm.c

We have the exact same comment inside intel_init_display. This is
a leftover from when we moved a lot of code from intel_display.c to
intel_pm.c.

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