drm/i915/selftests: Fix return in assert_mmap_offset()

The assert_mmap_offset() returns type bool so if we return an error
pointer that is "return true;" or success.  If we have an error, then
we should return false.

Fixes: 3d81d589

 ("drm/i915: Test exhaustion of the mmap space")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20200228141413.qfjf4abr323drlo4@kili.mountain