drm/i915: update intel_dp_voltage_max comment

Any comment containing "current Intel hardware supports" quickly
becomes obsolete, so remove it and let people discover the information
by looking at the function implementation.

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