drm/i915: Mask out the HPD irq bits before setting them individually.

To disable previously enabled HPD IRQs we need to reset them and
set the enabled ones individually.

Signed-off-by: Egbert Eich <eich@suse.de>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>