gallium: Correctly handle no config context creation

This patch fixes the following Piglit test:
spec@egl_mesa_configless_context@basic
It also fixes few test in a virgl guest.

v2: Evaluate the value of no_config (Ilia)

Suggested-by: Emil Velikov <emil.velikov@collabora.com>
Signed-off-by: Elie Tournier <elie.tournier@collabora.com>
Signed-off-by: Marek Olšák <marek.olsak@amd.com>
(cherry picked from commit 9179c745)