drm/msm: simplify gpu_busy callback

Move tracking and busy time calculation to msm_devfreq_get_dev_status.

Signed-off-by: Chia-I Wu <olvaffe@gmail.com>
Cc: Rob Clark <robdclark@chromium.org>
Link: https://lore.kernel.org/r/20220416003314.59211-2-olvaffe@gmail.com
Signed-off-by: Rob Clark <robdclark@chromium.org>