drm/msm: Pass the MMU domain index in struct msm_file_private

Pass the index of the MMU domain in struct msm_file_private instead
of assuming gpu->id throughout the submit path. This clears the way
to change ctx->aspace to a per-instance pagetable.

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