drm/i915/irq: relocate gmbus and dp aux irq handlers

Move gmbus and dp aux irq handlers to their respective files. It should
be up to them what to do with the irq, not the generic irq code.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/e825385fc03cb3d53c1f0b66712eea42dad69d59.1683219363.git.jani.nikula@intel.com