Rework set_parent core function so it can be called when the prepare lock
is already held by the caller.

This rework is done to ease the integration of the "protected" clock
functionality.
Acked-by: NLinus Walleij <linus.walleij@linaro.org>
Tested-by: NQuentin Schulz <quentin.schulz@free-electrons.com>
Tested-by: NMaxime Ripard <maxime.ripard@free-electrons.com>
Acked-by: NMichael Turquette <mturquette@baylibre.com>
Signed-off-by: NJerome Brunet <jbrunet@baylibre.com>
Signed-off-by: NMichael Turquette <mturquette@baylibre.com>
Link: lkml.kernel.org/r/20171201215200.23523-3-jbrunet@baylibre.com