drm/i915: Don't assign to struct drm_device.pdev

Using struct drm_device.pdev is deprecated. Don't assign it. Users
should upcast from struct drm_device.dev.

v6:
	* also fix the assignment in selftests in this patch (Chris)

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: 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>
Link: https://patchwork.freedesktop.org/patch/msgid/20210429105101.25667-5-tzimmermann@suse.de