drm/vmwgfx: Fix vmw_du_get_cursor_mob fencing of newly-created MOBs

The fencing of MOB creation used in vmw_du_get_cursor_mob was incompatible
with register-based device communication employed by this routine. As a
result cursor MOB creation was racy, leading to potentially broken/missing
mouse cursor on desktops using CursorMob device feature.

Fixes: 53bc3f6f ("drm/vmwgfx: Clean up cursor mobs")
Signed-off-by: Martin Krastev <martin.krastev@broadcom.com>
Reviewed-by: Maaz Mombasawala <maaz.mombasawala@broadcom.com>
Reviewed-by: Zack Rusin <zack.rusin@broadcom.com>
Signed-off-by: Zack Rusin <zack.rusin@broadcom.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240126200804.732454-5-zack.rusin@broadcom.com