drm/msm: Don't close VMAs on purge

Previously we'd also tear down the VMA, making the address space
available again.  But with drm_gpuvm conversion, this would require
holding the locks of all VMs the GEM object is mapped in.  Which is
problematic for the shrinker.

Instead just let the VMA hang around until the GEM object is freed.

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/661472/