drm/msm: Allow hardware clock gating to be toggled

There are some use cases wherein we need to turn off hardware clock
gating before reading certain registers. Modify the A5XX HWCG function
to allow user to enable or disable clock gating at will.

Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>
Signed-off-by: Rob Clark <robdclark@gmail.com>