drm/msm: Use drm_gpuvm types more

Most of the driver code doesn't need to reach in to msm specific fields,
so just use the drm_gpuvm/drm_gpuva types directly.  This should
hopefully improve commonality with other drivers and make the code
easier to understand.

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/661483/