drm/vmwgfx: Remove the repeated declaration

Function 'vmw_context_binding_list' is declared twice, remove the
repeated declaration.

Cc: VMware Graphics <linux-graphics-maintainer@vmware.com>
Cc: Roland Scheidegger <sroland@vmware.com>
Cc: Zack Rusin <zackr@vmware.com>
Signed-off-by: Shaokun Zhang <zhangshaokun@hisilicon.com>
Signed-off-by: Zack Rusin <zackr@vmware.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1621930170-54923-1-git-send-email-zhangshaokun@hisilicon.com