clk: ls1028a: fix a dereference of pointer 'parent' before a null check

clk: ls1028a: fix a dereference of pointer 'parent' before a null check

Currently the pointer 'parent' is being dereferenced before it is
being null checked. Fix this by performing the null check before
it is dereferenced.

Addresses-Coverity: ("Dereference before null check")
Fixes: d37010a3c162 ("clk: ls1028a: Add clock driver for Display output interface")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Link: https://lkml.kernel.org/r/20200203223736.99645-1-colin.king@canonical.com
Signed-off-by: Stephen Boyd <sboyd@kernel.org>