drm/msm: Collapse vma allocation and initialization

Now that we've dropped vram carveout support, we can collapse vma
allocation and initialization.  This better matches how things work
with drm_gpuvm.

Signed-off-by: Rob Clark <robdclark@chromium.org>
Signed-off-by: Rob Clark <robin.clark@oss.qualcomm.com>
Tested-by: Antonino Maniscalco <antomani103@gmail.com>
Reviewed-by: Antonino Maniscalco <antomani103@gmail.com>
Patchwork: https://patchwork.freedesktop.org/patch/661471/