msm: clock-8960: Store current HDMI PLL rate in clk->rate

This allows for a vdd_class, which reference clk->rate, to be used
with the HDMI PLL. It also removes the need to have a .get_rate
op implemented.

Change-Id: I1fd7eba16fa1e1af56513d400e58fdfdaff931e4
Signed-off-by: Matt Wagantall <mattw@codeaurora.org>