clk: qcom: gdsc: Don't override existing gdsc pd functions

In extreme cases an individual gdsc may wish to override the
power domain enable or disable callback functions for their own
purposes. Only set the generic gdsc callback if the function pointers
are not already set.

Acked-by: Rajendra Nayak <rnayak@codeaurora.org>
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>