piglit: upgrade to latest revision

Update piglit to latest git revision and update the branch name,
since the original one is no longer updated. Make sure the VK
tests are only enabled if VK is also enabled in PACKAGECONFIG,
and that this is opt-in, otherwise older systems fail to build.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Anuj Mittal <anuj.mittal@intel.com>
Cc: Richard Purdie <richard.purdie@linuxfoundation.org>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>