gpu: nvgpu: Fill in ACR header only once

gpu: nvgpu: Fill in ACR header only once

We call prepare_ucode_blob() once each time we un-railgate. We
allocate prepare the header for ACR ucode there, but the header
never gets freed.

Allocate and prepare the ACR header only once.

Change-Id: I948da8b47d6bb2fa021868d7038d2cc35eccb460
Signed-off-by: Terje Bergstrom <tbergstrom@nvidia.com>
Reviewed-on: http://git-master/r/729745
Reviewed-by: Seshendra Gadagottu <sgadagottu@nvidia.com>
Reviewed-by: Vijayakumar Subbu <vsubbu@nvidia.com>