drm/omap: dmm_tiler: Demote abusive use of kernel-doc format

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

 drivers/gpu/drm/omapdrm/omap_dmm_tiler.c:313: warning: Function parameter or member 'dmm' not described in 'dmm_txn_init'
 drivers/gpu/drm/omapdrm/omap_dmm_tiler.c:313: warning: Function parameter or member 'tcm' not described in 'dmm_txn_init'
 drivers/gpu/drm/omapdrm/omap_dmm_tiler.c:353: warning: Function parameter or member 'txn' not described in 'dmm_txn_append'
 drivers/gpu/drm/omapdrm/omap_dmm_tiler.c:353: warning: Function parameter or member 'area' not described in 'dmm_txn_append'
 drivers/gpu/drm/omapdrm/omap_dmm_tiler.c:353: warning: Function parameter or member 'pages' not described in 'dmm_txn_append'
 drivers/gpu/drm/omapdrm/omap_dmm_tiler.c:353: warning: Function parameter or member 'npages' not described in 'dmm_txn_append'
 drivers/gpu/drm/omapdrm/omap_dmm_tiler.c:353: warning: Function parameter or member 'roll' not described in 'dmm_txn_append'
 drivers/gpu/drm/omapdrm/omap_dmm_tiler.c:399: warning: Function parameter or member 'txn' not described in 'dmm_txn_commit'
 drivers/gpu/drm/omapdrm/omap_dmm_tiler.c:399: warning: Function parameter or member 'wait' not described in 'dmm_txn_commit'

Cc: Tomi Valkeinen <tomi.valkeinen@ti.com>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Rob Clark <rob@ti.com>
Cc: Andy Gross <andy.gross@ti.com>
Cc: dri-devel@lists.freedesktop.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201106214949.2042120-10-lee.jones@linaro.org