drm/i915 & drm/xe: save struct drm_device to drvdata

drm/i915 & drm/xe: save struct drm_device to drvdata

In the future, the display code shall not have any idea about struct
xe_device or struct drm_i915_private, but will need to get at the struct
drm_device via drvdata. Store the struct drm_device pointer to drvdata
instead of the driver specific pointer.

Avoid passing NULL to container_of() via to_i915()/to_xe_device(). (It
does return NULL for NULL pointers when the offset happens to be 0, but
otherwise returns garbage pointers for NULL.)

Reviewed-by: Gustavo Sousa <gustavo.sousa@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/946805b32e38d4785880cc7857e01e6a309126a9.1724942754.git.jani.nikula@intel.com
Signed-off-by: Jani Nikula <jani.nikula@intel.com>