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: 9655441 ("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

Author: en4bz

drivers/gpu/drm/vmwgfx/vmwgfx_kms.c

@@ -766,13 +766,15 @@ static struct drm_framebuffer *vmw_kms_fb_create(struct drm_device *dev,
 		return ERR_PTR(ret);
 	}
 
-	ttm_bo_reserve(&bo->tbo, false, false, NULL);
-	ret = vmw_bo_dirty_add(bo);
-	if (!ret && surface && surface->res.func->dirty_alloc) {
-		surface->res.coherent = true;
-		ret = surface->res.func->dirty_alloc(&surface->res);
+	if (bo) {
+		ttm_bo_reserve(&bo->tbo, false, false, NULL);
+		ret = vmw_bo_dirty_add(bo);
+		if (!ret && surface && surface->res.func->dirty_alloc) {
+			surface->res.coherent = true;
+			ret = surface->res.func->dirty_alloc(&surface->res);
+		}
+		ttm_bo_unreserve(&bo->tbo);
 	}
-	ttm_bo_unreserve(&bo->tbo);
 
 	return &vfb->base;
 }