egl/dri2: short-circuit dri2_make_current when possible

egl/dri2: short-circuit dri2_make_current when possible

If an application calls eglMakeCurrent with the same context and the same
draw and read surfaces as the current ones, there is no need to go
through all the work of unbinding/flushing the old context and binding
the new one.

While the EGL spec is a bit ambiguous here, it seems that the implicit
flush of the outgoing context only needs to be done when the context is
actually changed.

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>