* Moved all instances of gDeviceType to imageHelper.cpp
* Missed one instance of gDeviceType
* Removed all instances of extern cl_device_type gDeviceType, except in imageHelpers.h
Some of the setup functionality is already there in the test harness, so
use that and remove the duplicated code from within the suite.
Signed-off-by: Radek Szymanski <radek.szymanski@arm.com>