drm/msm: Split out get_freq() helper

In the next patch, it grows a bit more, so lets not duplicate the logic
in multiple places.

Signed-off-by: Rob Clark <robdclark@chromium.org>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Link: https://lore.kernel.org/r/20210726144653.2180096-3-robdclark@gmail.com
Signed-off-by: Rob Clark <robdclark@chromium.org>