drm/nouveau: Don't include drm_legacy.h

Nouveau does not use DRM legacy code. Remove the rsp. include
statement.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210516185937.5644-3-tzimmermann@suse.de