drm/i915: Drop all references to DRM IRQ midlayer

Remove all references to DRM's IRQ midlayer. i915 uses Linux' interrupt
functions directly.

v2:
	* also remove an outdated comment
	* move IRQ fix into separate patch
	* update Fixes tag (Daniel)

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Fixes: b318b824 ("drm/i915: Nuke drm_driver irq vfuncs")
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: intel-gfx@lists.freedesktop.org
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20210701173618.10718-3-tzimmermann@suse.de