drm/msm/adreno: Prevent unclocked access when retrieving timestamps

msm_gpu's get_timestamp() op (called by the MSM_GET_PARAM ioctl) can
result in register accesses. We need our power domain and clocks to
be active for that. Make sure they are enabled here.

Signed-off-by: Archit Taneja <architt@codeaurora.org>
Signed-off-by: Rob Clark <robdclark@gmail.com>