drm/i915: Don't clear drvdata in ->release

For two reasons:

- The driver core clears this already for us after we're unloaded in
  __device_release_driver().

- It's way too late, the drm_device ->release callback might massively
  outlive the underlying physical device, since a drm_device can be
  kept alive by open drm_file or well really anything else userspace
  is still hanging onto. So if we clear this ourselves, we should
  clear it in the pci ->remove callback, not in the drm_device
  ->release callback.

Looking at git history this was fixed in the driver core with

commit 0998d06310
Author: Hans de Goede <hdegoede@redhat.com>
Date:   Wed May 23 00:09:34 2012 +0200

    device-core: Ensure drvdata = NULL when no driver is bound

v2: Cite the core fix in the commit message (Chris).

v3: Fix commit message and unused variable warning (Jani).

Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200323144950.3018436-3-daniel.vetter@ffwll.ch

drivers/gpu/drm/i915/i915_drv.c

@@ -1392,13 +1392,8 @@ i915_driver_create(struct pci_dev *pdev, const struct pci_device_id *ent)
 
 static void i915_driver_destroy(struct drm_i915_private *i915)
 {
-	struct pci_dev *pdev = i915->drm.pdev;
-
 	drm_dev_fini(&i915->drm);
 	kfree(i915);
-
-	/* And make sure we never chase our dangling pointer from pci_dev */
-	pci_set_drvdata(pdev, NULL);
 }
 
 /**