drm/msm/sde: clk_set rate on core_clk instead of core_clk_src

core_clk_src handles are not required and will be removed.
This patch moves core_clk_src set rate calls to core_clk.

Change-Id: Icdcc2b6e8b9aaa7203015e7d1b4714bf68e4842f
Signed-off-by: Dhaval Patel <pdhaval@codeaurora.org>