Documentation: drm: Remove dangling pointer from drm-mm.rst

Fixes htmldocs build error:
Error: Cannot open file ../drivers/gpu/drm/drm_global.c

Fixes: 2bb42410b1bd ("drm: Remove drm_global.{c,h} v2")
Cc: Thomas Zimmermann <[e-mail]>
Cc: Christian König <[e-mail]>
Cc: Junwei Zhang <[e-mail]>
Cc: Alex Deucher <[e-mail]>
Cc: Maarten Lankhorst <[e-mail]>
Cc: Maxime Ripard <[e-mail]>
Cc: Sean Paul <[e-mail]>
Cc: David Airlie <[e-mail]>
Cc: Christian Koenig <[e-mail]>
Cc: Huang Rui <[e-mail]>
Cc: [e-mail]
Signed-off-by: Sean Paul <[e-mail]>
Reviewed-by: Christian König <[e-mail]>
Signed-off-by: Dave Airlie <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]

Documentation/gpu/drm-mm.rst

···
 
 See the radeon_ttm.c file for an example of usage.
 
-.. kernel-doc:: drivers/gpu/drm/drm_global.c
-   :export:
-
 
 The Graphics Execution Manager (GEM)
 ====================================