drm/irq: remove check on dev->dev_private

drm/irq: remove check on dev->dev_private

There is no real reason to require drivers to set and use
dev->dev_private. Indeed, the current recommendation, as documented in
drm_device.h, is to embed struct drm_device in the per-device struct
instead of using dev_private.

Remove the requirement for dev_private to have been set to indicate
driver initialization.

For background, quoting Daniel Vetter:

Now there might be some hilarious races this papers over, but:

- Proper drivers should only call drm_dev_register once everything is
  set up, including this stuff here. No race possible with anything else
  really.

- Slightly more wobbly drivers, including the legacy ones, all use
  drm_global_mutex. This was the former BKL, which means that it was
  impossible for soeone to go through the load/unload/reload (between
  lastclose and firstopen) paths and also run the ioctl. But the ioctl
  had to be made unlocked because blocking there killed X:

	 commit d7cc1f3a032bafbb29321f60fc8e7514187f2f21
	 Author: Ilija Hadzic <ihadzic@research.bell-labs.com>
	 Date:   Mon Oct 31 17:46:18 2011 -0400

	     drm: do not sleep on vblank while holding a mutex

  The even more legacy DRM_CONTROL ioctl stayed fully locked. But the
  file open/close paths are still fully locked, and that's the only
  place legacy drivers should call drm_irq_install/uninstall, so should
  all still be fully ordered and protected and happy.

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Daniel Vetter <daniel@ffwll.ch>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200211144753.3175-1-jani.nikula@intel.com