drm/i915/selftests: Move local mock_ggtt allocations to the heap

This struct appears quite large and pushes our stack frame over
1024 bytes -- too high for conservative setups. So move the mock_ggtt
struct to the heap.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190217202518.24730-1-chris@chris-wilson.co.uk