drm/i915: s/hotplugt_status_gen4/hotplug_status_g4x/

We specifically exclude original gen4 (i.e. i965g/gm), so update the
naming for consistency. Spotted while reviewing related code due to a
report from Jesse about byt needing again different values.

v2: g4x, not gm45 since this also applies to the desktop version.

Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>