clk: qcom: clk-rcg2: Declare the rcg struct in clk_gfx3d_src_determine_rate

Declare the rcg structure before referencing it in the
function.

Change-Id: Ife9fdabf20b242a93635532b9a0c4cacc8b88210
Signed-off-by: Deepak Katragadda <dkatraga@codeaurora.org>