drm/vmwgfx: fix all kernel-doc warnings in stdu

drm/vmwgfx: fix all kernel-doc warnings in stdu

kernel test robot reports one kernel-doc warning in stdu, but
running scripts/kernel-doc in -Wall mode reports several more,
so fix all of them at one time:

vmwgfx_stdu.c:76: warning: Excess struct member 'transfer' description in 'vmw_stdu_dirty'
vmwgfx_stdu.c:103: warning: missing initial short description on line:
 * struct vmw_screen_target_display_unit
vmwgfx_stdu.c:215: warning: No description found for return value of 'vmw_stdu_bind_st'
vmwgfx_stdu.c:320: warning: No description found for return value of 'vmw_stdu_destroy_st'
vmwgfx_stdu.c:551: warning: No description found for return value of 'vmw_kms_stdu_readback'
vmwgfx_stdu.c:719: warning: No description found for return value of 'vmw_kms_stdu_surface_dirty'
vmwgfx_stdu.c:895: warning: No description found for return value of 'vmw_stdu_primary_plane_prepare_fb'
vmwgfx_stdu.c:1470: warning: No description found for return value of 'vmw_stdu_init'

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202312150347.5icezNlK-lkp@intel.com/
Cc: Zack Rusin <zackr@vmware.com>
Cc: VMware Graphics Reviewers <linux-graphics-maintainer@vmware.com>
Cc: dri-devel@lists.freedesktop.org
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Maxime Ripard <mripard@kernel.org>
Cc: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Zack Rusin <zack.rusin@broadcom.com>
Signed-off-by: Zack Rusin <zack.rusin@broadcom.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231215234102.16574-1-rdunlap@infradead.org