drm/msm/hdmi: set hdp clock rate before prepare_enable

The clock driver usually complains when a clock is being prepared
before setting its rate. It is the case here for "core_clk" which
needs to be set at 19.2 MHz before we attempt a prepare_enable().

Signed-off-by: Stephane Viau <sviau@codeaurora.org>
Signed-off-by: Rob Clark <robdclark@gmail.com>