drm/doc: Relax fdinfo string constraints

The restriction about no whitespace, etc, really only applies to the
usage of strings in keys.  Values can contain anything (other than
newline).

Signed-off-by: Rob Clark <robdclark@chromium.org>
Acked-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Acked-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Neil Armstrong <neil.armstrong@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20230524155956.382440-8-robdclark@gmail.com