lib/drmtest: Use igt_assert in the do_ioctl/do_or_die helpers

lib/drmtest: Use igt_assert in the do_ioctl/do_or_die helpers

abort results in "crash" test results in the piglit runner, igt gives
us neater output.

The downside is that we can now only use these in igt tests, but
that's already the case (and given their placement in drmtest.h their
intended use-case, too).

Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>