drm/i915: clean up HPD IRQ debug printing

Atm, we don't print these events for all platforms and for VLV/G4X we
also print them for DP AUX completion events which is unnecessary spam.
Fix both issues.

Signed-off-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>