drm/i915: storm detection documentation update

Update the hotplug documentation to explain that hotplug storm
is not expected for Display port panels and hence is not handled
in current code.

v2: update the statements as recommended by Daniel

Signed-off-by: Sivakumar Thulasimani <sivakumar.thulasimani@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>