egl/dri2: remove superfluous flush when changing the context

egl/dri2: remove superfluous flush when changing the context

The flush of the outgoing GL context, as required by the EGL spec for
eglMakeCurrent and extended by KHR_context_flush_control, is already
performed in the unbindContext call. There is no need to pierce through
the layers and unconditionally call glFlush() here.

Getting the out-fence FD for explicit fencing needs to move behind the
unbindContext, to make sure we are getting the fence for the most
recently flushed commands.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Reviewed-by: Adam Jackson <ajax@redhat.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/14379>