tests/i915/gem_mmap_*: Add NULL to ptrace() calls

In FreeBSD, the ptrace() function cannot take a variable
length list of arguments. This patch adds NULL in the @data
parameter to allow for compilation on FreeBSD systems.

Signed-off-by: Jake Freeland <jfree@freebsd.org>
Reviewed-by: Kamil Konieczny <kamil.konieczny@linux.intel.com>