drm: Include <drm/drm_auth.h>

One of the source files includes <drm/drm_auth.h> via <drm/drm_legacy.h>,
which will be removed. Include drm_auth.h directly.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: David Airlie <airlied@gmail.com>
Reviewed-by: Daniel Vetter <daniel@ffwll.ch>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231122122449.11588-4-tzimmermann@suse.de