drm/vmwgfx: Fix KMS with 3D on HW version 10

drm/vmwgfx: Fix KMS with 3D on HW version 10

HW version 10 does not have GB Surfaces so there is no backing buffer for
surface backed FBs. This would result in a nullptr dereference and crash
the driver causing a black screen.

Fixes: 965544150d1c ("drm/vmwgfx: Refactor cursor handling")
Signed-off-by: Ian Forbes <ian.forbes@broadcom.com>
Reviewed-by: Zack Rusin <zack.rusin@broadcom.com>
Signed-off-by: Zack Rusin <zack.rusin@broadcom.com>
Link: https://patch.msgid.link/20251114203703.1946616-1-ian.forbes@broadcom.com