drm/i915: fold the hpd_irq_setup call into intel_hpd_irq_handler

We already have a vfunc for this (and other parts of the hpd storm
handling code already use it).

v2: Rebase on top of the i965g/gm sdvo hpd fix.

Cc: Egbert Eich <eich@suse.de>
Reviewed-by: Egbert Eich <eich@suse.de>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>