drm/gpuvm: Add locking helpers

For UNMAP/REMAP steps we could be needing to lock objects that are not
explicitly listed in the VM_BIND ioctl in order to tear-down unmapped
VAs.  These helpers handle locking/preparing the needed objects.

Note that these functions do not strictly require the VM changes to be
applied before the next drm_gpuvm_sm_map_lock()/_unmap_lock() call.  In
the case that VM changes from an earlier drm_gpuvm_sm_map()/_unmap()
call result in a differing sequence of steps when the VM changes are
actually applied, it will be the same set of GEM objects involved, so
the locking is still correct.

v2: Rename to drm_gpuvm_sm_*_exec_locked() [Danilo]
v3: Expand comments to show expected usage, and explain how the usage
    is safe in the case of overlapping driver VM_BIND ops.

Signed-off-by: Rob Clark <[email protected]>
Tested-by: Antonino Maniscalco <[email protected]>
Reviewed-by: Antonino Maniscalco <[email protected]>
Acked-by: Danilo Krummrich <[email protected]>
Patchwork: https://patchwork.freedesktop.org/patch/661458/

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