drm/etnaviv: move idle mapping reaping into separate function

The same logic is already used in two different places and now
it will also be needed outside of the compilation unit, so split
it into a separate function.

Cc: stable@vger.kernel.org # 5.19
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Reviewed-by: Guido Günther <agx@sigxcpu.org>