Use a stable tag instead of some random commit from mainstream
development, to avoid unexpected build failures.

This fixes:

    CC       virglrenderer.lo
  virglrenderer.c: In function 'virgl_has_gl_colorspace':
  virglrenderer.c:208:11: error: implicit declaration of function 'virgl_has_egl_khr_gl_colorspace' [-Werror=implicit-function-declaration]
             virgl_has_egl_khr_gl_colorspace(egl_info));
             ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  virglrenderer.c:208:43: error: 'egl_info' undeclared (first use in this function)
             virgl_has_egl_khr_gl_colorspace(egl_info));
                                             ^~~~~~~~
  virglrenderer.c:208:43: note: each undeclared identifier is reported only once for each function it appears in
  cc1: some warnings being treated as errors

As of this commit 'git virglrenderer-0.7.0' is the last stable tag.
(virglrenderer commit breaking: fb4f7577f7ef)

Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>