drm/msm: Split out map/unmap ops

With async VM_BIND, the actual pgtable updates are deferred.
Synchronously, a list of map/unmap ops will be generated, but the
actual pgtable changes are deferred.  To support that, split out
op handlers and change the existing non-VM_BIND paths to use them.

Note in particular, the vma itself may already be destroyed/freed
by the time an UNMAP op runs (or even a MAP op if there is a later
queued UNMAP).  For this reason, the op handlers cannot reference
the vma pointer.

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/661516/