drm: drop DRM_AUTH requirement from AUTH_MAGIC ioctl

Currently only an authenticated master can authenticate another client.

In practise the client can only be master if CAP_SYS_ADMIN is present,
although having the CAP also sets the client as authenticated.

Thus DRM_AUTH in AUTH_MAGIC's "DRM_AUTH | DRM_MASTER" is superfluous.

Notices while working on IGT tests.

Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20190114084305.15141-1-emil.l.velikov@gmail.com