drm/vmwgfx/vmwgfx_stdu: Add some missing param/member descriptions

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

 drivers/gpu/drm/vmwgfx/vmwgfx_stdu.c:77: warning: Function parameter or member 'pitch' not described in 'vmw_stdu_dirty'
 drivers/gpu/drm/vmwgfx/vmwgfx_stdu.c:125: warning: Function parameter or member 'content_fb_type' not described in 'vmw_screen_target_display_unit'
 drivers/gpu/drm/vmwgfx/vmwgfx_stdu.c:125: warning: Function parameter or member 'display_width' not described in 'vmw_screen_target_display_unit'
 drivers/gpu/drm/vmwgfx/vmwgfx_stdu.c:125: warning: Function parameter or member 'display_height' not described in 'vmw_screen_target_display_unit'
 drivers/gpu/drm/vmwgfx/vmwgfx_stdu.c:125: warning: Function parameter or member 'cpp' not described in 'vmw_screen_target_display_unit'
 drivers/gpu/drm/vmwgfx/vmwgfx_stdu.c:682: warning: Function parameter or member 'user_fence_rep' not described in 'vmw_kms_stdu_dma'

Cc: VMware Graphics <linux-graphics-maintainer@vmware.com>
Cc: Roland Scheidegger <sroland@vmware.com>
Cc: Zack Rusin <zackr@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/20210115181313.3431493-41-lee.jones@linaro.org