[ALPS05444578] ION: fix "pointer is NULL" issue

[Root cause]
When ion get dma_buf by file->private_data in ion_drv_file_to_buffer func,
it maybe NULL and use it to lead to KE.

[Solution]
After get dma_buf by file->private_data, we need to check if it is NULL
and then handle it.

In addition, delete unnecessary check by ion_drv_file_to_buffer.

MTK-Commit-Id: 455b5c0af23178ef21ca95d23668be93c633bf30

Change-Id: I6ea135c9eaed9520440a59dd4c662a9bb8691bc7
Signed-off-by: Guangming Cao <Guangming.Cao@mediatek.com>
CR-Id: ALPS05444578
Feature: ION