drm/msm: gpu: Call pmruntime ops before attaching to the smmu

The arm-smmu driver has runtime pm enabled and the devicelinks
between smmu (suppliers) and masters (gpu, mdss etc) is
set. So with the devicelink in place, it will be the
responsiblity of the masters to enable the power to the
suppliers before using them. So call pmruntime_get_sync
in gpu before trying to attach to its smmu and relinquish
it once its done.

Note that similar thing has to be done on other places
as well (if found), when the master wants its supplier
to be powered.

Signed-off-by: Sricharan R <sricharan@codeaurora.org>