fbdev/core: Use list_for_each_entry() helper

Convert list_for_each() to list_for_each_entry() so that the pos
list_head pointer and list_entry() call are no longer needed, which
can reduce a few lines of code. No functional changed.

Signed-off-by: Jinjie Ruan <ruanjinjie@huawei.com>
Signed-off-by: Helge Deller <deller@gmx.de>