drm/i915/selftests: Use vma_lookup() in __igt_mmap()

drm/i915/selftests: Use vma_lookup() in __igt_mmap()

vma_lookup() will look up the vma at a specific address.  find_vma()
will start the search for a specific address and continue upwards.  This
fixes an issue with the selftest as the returned vma may not be the
newly created vma, but simply the vma at a higher address.

Fixes: 6fedafacae1b (drm/i915/selftests: Wrap vm_mmap() around GEM
objects
Signed-off-by: Liam R. Howlett <Liam.Howlett@Oracle.com>