drm: msm: a6xx: use dev_pm_opp_set_bw to scale DDR

This patches replaces the previously used static DDR vote and uses
dev_pm_opp_set_bw() to scale GPU->DDR bandwidth along with scaling
GPU frequency. Also since the icc path voting is handled completely
in the opp driver, remove the icc_path handle and its usage in the
drm driver.

Signed-off-by: Sharat Masetty <smasetty@codeaurora.org>
Signed-off-by: Akhil P Oommen <akhilpo@codeaurora.org>
Signed-off-by: Rob Clark <robdclark@chromium.org>