gpu: ion: Rename request/release region

request_region and release_region are macro names defined
in the linux kernel. Under some circumstances, the C compiler
can't differentiate between the macro name and the field name.
Changing the field name is the easiest way to prevent this
problem.

Change-Id: I2c8d61bdaa20e332e0215f0bb3237e8332f0f3ac
Signed-off-by: Laura Abbott <lauraa@codeaurora.org>