msm: kgsl: Set only valid values in kgsl_ft_policy

Only write valid GFT policy bits in the ft_policy bitfield
from sysfs.

Change-Id: Ib479c01abead57c48199e7bcde4435b4ccaa42e4
Signed-off-by: Harsh Vardhan Dwivedi <hdwivedi@codeaurora.org>
Signed-off-by: Tarun Karra <tkarra@codeaurora.org>