drm/i915: fold the no-irq check into intel_hpd_irq_handler

The usual pattern for our sub-function irq_handlers is that they check
for the no-irq case themselves. This results in more streamlined code
in the upper irq handlers.

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>