drm/doc: Update IGT documentation

The IGT documentation in this page is telling us to build it using
make. According to commit 67993c1 ("automake: Point builders at
meson") from the IGT project, this is deprecated and IGT should be
built with meson. Instead of having a documentation for IGT in this
page, point to their GitLab README, which should always be up to
date.

Signed-off-by: Leandro Ribeiro <leandrohr@riseup.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20191030011211.47942-1-leandrohr@riseup.net