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And again make sure it's a no-op for modern drivers. Another case of
dev->struct_mutex gone for modern drivers!

Note that the entirety of the legacy addmap interface is now protected
by DRIVER_MODESET. Note that just auditing kernel code is not enough,
since userspace loves to set up legacy maps on it's own for various
things - with ums userspace and kernel space share control over
resources.

v2: Also add a DRIVER_* check like for all other maps functions to
really short-circuit the code. And give drm_legacy_rmmap used by the
dev unregister code the same treatment.

v3:
- remove redundant return; (Alex, Chris)
- don't special case nouveau with DRIVER_KMS_LEGACY_CONTEXT.

v4: Again special case nouveau. The problem is not directly in the
ddx, but that it calls dri1 functions from the X server. And those do
call drmAddMap. Fixed only in

    commit b1a630b48210d6a3c44994fce1b73273000ace5c
    Author: Dave Airlie <airlied@redhat.com>
    Date:   Wed Nov 7 14:45:14...