piglit: upgrade to more recent srvrev

piglit: upgrade to more recent srvrev

Upgrade to 126c7d from December 2014.  Still not the latest but it's got no
added build dependencies and importantly handles installs out of the box instead
of us having to hack and kludge, and builds with cmake 3.x.

The usage has changed: now there's a "piglit" binary that has run/resume/summary
subcommands, for example:

$ piglit run quick quick-results/

For reference the quick test suite on a IvyBridge NUC results in:

[27714/27714] crash: 3, fail: 326, pass: 18275, skip: 9107, warn: 3 -

(From OE-Core rev: 562c8907d6afcf50695f8964cdebd05fc1085742)

Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>