drm/vmwgfx/vmwgfx_ldu: Supply descriptions for 'state' function parameter

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

 drivers/gpu/drm/vmwgfx/vmwgfx_ldu.c:55: warning: cannot understand function prototype: 'struct vmw_legacy_display_unit '
 drivers/gpu/drm/vmwgfx/vmwgfx_ldu.c:218: warning: Function parameter or member 'state' not described in 'vmw_ldu_crtc_atomic_enable'
 drivers/gpu/drm/vmwgfx/vmwgfx_ldu.c:228: warning: Function parameter or member 'state' not described in 'vmw_ldu_crtc_atomic_disable'

Cc: VMware Graphics <linux-graphics-maintainer@vmware.com>
Cc: Roland Scheidegger <sroland@vmware.com>
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: Zack Rusin <zackr@vmware.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210115181313.3431493-14-lee.jones@linaro.org