clk: qcom: gdsc: Fix issue with dereferencing NULL pointer in gdsc APIs

Fix the issue with deferencing the rdev structure for
the GDSC prior to it being populated. Replace the
qcom,vote-parent-supply-voltage property with
vdd_parent-supply and use the regulator handle that's
returned from calling regulator_get for the parent
regulator directly.

Change-Id: I9a80cbeb4fd88b718e6e7509d8868d965253ae5e
Signed-off-by: Deepak Katragadda <dkatraga@codeaurora.org>