drm/doc: Fix missing @ctx documentation

Forgot to add this :(

Fixes: 19315294 ("drm: Add acquire ctx parameter to ->plane_disable")
Cc: Harry Wentland <harry.wentland@amd.com>
Reviewed-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170413074007.7620-1-daniel.vetter@ffwll.ch