drm/i915/gt: Provide a utility to create a scratch buffer

Primarily used by selftests, but also by runtime debugging of engine
w/a, is a routine to create a temporarily bound buffer for readback.
Almagamate the duplicated routines into one.

Suggested-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201219020343.22681-2-chris@chris-wilson.co.uk