clk: Remove global clk traversal on fetch parent index

It's not required to traverse the entire clk tree when the parents array
contains a NULL value. We already have the parent clk_core pointer, so
we can just compare the parent->name and parent_names[i] pointers.

This can be a substantial power improvement in cases where the parent
clk isn't known and that clk is never registered, because a mux having
an unregistered parent name may traverse the clk tree on every
clk_set_rate() call in clk_mux_determine_rate_flags(). This can happen
hundreds of times a second for CPU clks.

This patch is the combination of reverting commit 470b5e2f ("clk:
simplify clk_fetch_parent_index() function") and optimizing the
resulting code to never call __clk_lookup() because we already have the
clk_core pointer we're looking for. That optimization went unnoticed
even after commit da0f0b2c ("clk: Correct lookup logic in
clk_fetch_parent_index()") tried to optimize this path.

Signed-off-by: Derek Basehore <dbasehore@chromium.org>
[sboyd@kernel.org: More description in commit text]
Signed-off-by: Stephen Boyd <sboyd@kernel.org>