drm/i915: don't prevent CPU idle states

Commit 9ee32fea unconditionally prevents the CPU from entering idle states
until intel_dp_aux_ch completes for the first time, which never happens on my
DisplayPort-less intel gfx, causing the CPU to get rather hot.

Signed-off-by: Tomas Janousek <tomi@nomi.cz>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>