drm/vmwgfx/vmwgfx_scrn: Demote unworthy kernel-doc headers and update others

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

 drivers/gpu/drm/vmwgfx/vmwgfx_scrn.c:90: warning: cannot understand function prototype: 'struct vmw_screen_object_unit '
 drivers/gpu/drm/vmwgfx/vmwgfx_scrn.c:122: warning: Function parameter or member 'dev_priv' not described in 'vmw_sou_fifo_create'
 drivers/gpu/drm/vmwgfx/vmwgfx_scrn.c:122: warning: Function parameter or member 'sou' not described in 'vmw_sou_fifo_create'
 drivers/gpu/drm/vmwgfx/vmwgfx_scrn.c:122: warning: Function parameter or member 'x' not described in 'vmw_sou_fifo_create'
 drivers/gpu/drm/vmwgfx/vmwgfx_scrn.c:122: warning: Function parameter or member 'y' not described in 'vmw_sou_fifo_create'
 drivers/gpu/drm/vmwgfx/vmwgfx_scrn.c:122: warning: Function parameter or member 'mode' not described in 'vmw_sou_fifo_create'
 drivers/gpu/drm/vmwgfx/vmwgfx_scrn.c:168: warning: Function parameter or member 'dev_priv' not described in 'vmw_sou_fifo_destroy'
 drivers/gpu/drm/vmwgfx/vmwgfx_scrn.c:168: warning: Function parameter or member 'sou' not described in 'vmw_sou_fifo_destroy'
 drivers/gpu/drm/vmwgfx/vmwgfx_scrn.c:283: warning: Function parameter or member 'state' not described in 'vmw_sou_crtc_atomic_enable'
 drivers/gpu/drm/vmwgfx/vmwgfx_scrn.c:293: warning: Function parameter or member 'state' not described in 'vmw_sou_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/20210115181313.3431493-30-lee.jones@linaro.org