drm/vc4/vc4_debugfs: Demote non-conformant kernel-doc headers

Fixes the following W=1 kernel build warning(s):

 drivers/gpu/drm/vc4/vc4_debugfs.c:25: warning: Function parameter or member 'minor' not described in 'vc4_debugfs_init'
 drivers/gpu/drm/vc4/vc4_debugfs.c:62: warning: Function parameter or member 'dev' not described in 'vc4_debugfs_add_file'
 drivers/gpu/drm/vc4/vc4_debugfs.c:62: warning: Function parameter or member 'name' not described in 'vc4_debugfs_add_file'
 drivers/gpu/drm/vc4/vc4_debugfs.c:62: warning: Function parameter or member 'show' not described in 'vc4_debugfs_add_file'
 drivers/gpu/drm/vc4/vc4_debugfs.c:62: warning: Function parameter or member 'data' not described in 'vc4_debugfs_add_file'

Cc: Eric Anholt <eric@anholt.net>
Cc: Maxime Ripard <mripard@kernel.org>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: dri-devel@lists.freedesktop.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20201116174112.1833368-41-lee.jones@linaro.org