piglit (script): Don't set PIGLIT_SOURCE_DIR if path doesn't exist

Otherwise, the user gets confronted by this cryptic import error
message:

  ImportError: No module named framework.programs.run

Now, the user gets this slightly more helpful message:

  Exception: failed to detect PIGLIT_SOURCE_DIR

Signed-off-by: Chad Versace <chad.versace@linux.intel.com>
piglit (script): Fix when installed with LIBDIR=lib64

When installing piglit with CMAKE_INSTALL_LIBDIR="lib64", the piglit
script failed to detect PIGLIT_SOURCE_DIR.  The detection code expected
CMAKE_INSTALL_LIBDIR="lib".

This patch fixes the piglit script to inspect "lib64" too.

Signed-off-by: Chad Versace <chad.versace@linux.intel.com>