drm/locking: fix drm_modeset_acquire_ctx kernel-doc

The stack_depot member was added without kernel-doc, leading to below
warning. Fix it.

./include/drm/drm_modeset_lock.h:74: warning: Function parameter or
member 'stack_depot' not described in 'drm_modeset_acquire_ctx'

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Fixes: cd06ab2f ("drm/locking: add backtrace for locking contended locks without backoff")
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Tested-by: Stephen Rothwell <sfr@canb.auug.org.au>
Link: https://patchwork.freedesktop.org/patch/msgid/20220120094856.3004147-1-jani.nikula@intel.com