drm/doc: Include new drm-compute documentation

Commit b168ed458dde ("kernel/cgroup: Add "dmem" memory accounting
cgroup") introduced a new documentation file, but didn't link it
anywhere. It was thus triggering a documentation build warning. Make
sure it's included as part of the DRM documentation.

Fixes: b168ed458dde ("kernel/cgroup: Add "dmem" memory accounting cgroup")
Reported-by: Stephen Rothwell <[e-mail]>
Closes: https://lore.kernel.org/r/[e-mail]/
Acked-by: Tejun Heo <[e-mail]>
Reviewed-by: Simona Vetter <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]
Signed-off-by: Maxime Ripard <[e-mail]>

Documentation/gpu/index.rst

···
    drm-usage-stats
    driver-uapi
    drm-client
+   drm-compute
    drivers
    backlight
    vga-switcheroo