drm/msm/gpu: Map the ringbuffer in the iova at create time

For reasons that I'm sure made perfect sense at the time we were
opting to defer the iova alloc / pin on the ringbuffer until HW
init time so when we moved to iova reference counting we ended
up adding a reference count every time the hardware started.
Not that it mattered (because the ring is always around) but
it did make the debug output look odd. Allocate and pin the iova
at create time instead.

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