dri_util: when overriding, always reset the core version

dri_util: when overriding, always reset the core version

This way we won't fail when validating just because we may have a non
overriden core version that is lower than the requested one, even when
the compat version is high enough.

For example, running glcts from VK-GL-CTS with i965, this will
succeed:

$ MESA_GL_VERSION_OVERRIDE=4.6 ./glcts --deqp-case=KHR-GL46.info.vendor

While, this will fail:

$ MESA_GL_VERSION_OVERRIDE=4.6COMPAT ./glcts --deqp-case=KHR-GL46.info.vendor

Fixes: 464c56d3d5c ("dri_util: Use
_mesa_override_gl_version_contextless")

Cc: Ian Romanick <ian.d.romanick@intel.com>
Cc: Tapani Pälli <tapani.palli@intel.com>
Cc: Marek Olšák <marek.olsak@amd.com>
Signed-off-by: Andres Gomez <agomez@igalia.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Tapani Pälli <tapani.palli@intel.com>