drm/vmwgfx: Fix incorrect command header offset at restart

Sometimes it appears like the device modifies the command header offset
member. So explicitly clear it when restarting after an error.

Signed-off-by: Thomas Hellstrom <[e-mail]>
Reviewed-by: Sinclair Yeh <[e-mail]>

drivers/gpu/drm/vmwgfx/vmwgfx_cmdbuf.c

···
 		entry->cmd += new_start_offset;
 		cb_hdr->length -= new_start_offset;
 		cb_hdr->errorOffset = 0;
+		cb_hdr->offset = 0;
 		list_add_tail(&entry->list, &restart_head[entry->cb_context]);
 		man->ctx[entry->cb_context].block_submission = true;
 	}