drm/vmwgfx: remove unused struct 'vmw_stdu_dma'

'vmw_stdu_dma' is unused since
commit 39985eea ("drm/vmwgfx: Abstract placement selection")
Remove it.

Signed-off-by: Dr. David Alan Gilbert <linux@treblig.org>
Signed-off-by: Zack Rusin <zack.rusin@broadcom.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240517232858.230860-1-linux@treblig.org