drm/gpuvm: Wrap drm_gpuvm_sm_map_exec_lock() expected usage in literal code block

Stephen Rothwell reports multiple indentation warnings when merging
drm-msm tree:

Documentation/gpu/drm-mm:506: ./drivers/gpu/drm/drm_gpuvm.c:2445: ERROR: Unexpected indentation. [docutils]
Documentation/gpu/drm-mm:506: ./drivers/gpu/drm/drm_gpuvm.c:2447: WARNING: Block quote ends without a blank line; unexpected unindent. [docutils]
Documentation/gpu/drm-mm:506: ./drivers/gpu/drm/drm_gpuvm.c:2451: WARNING: Definition list ends without a blank line; unexpected unindent. [docutils]
Documentation/gpu/drm-mm:506: ./drivers/gpu/drm/drm_gpuvm.c:2452: WARNING: Definition list ends without a blank line; unexpected unindent. [docutils]
Documentation/gpu/drm-mm:506: ./drivers/gpu/drm/drm_gpuvm.c:2456: ERROR: Unexpected indentation. [docutils]
Documentation/gpu/drm-mm:506: ./drivers/gpu/drm/drm_gpuvm.c:2457: WARNING: Definition list ends without a blank line; unexpected unindent. [docutils]
Documentation/gpu/drm-mm:506: ./drivers/gpu/drm/drm_gpuvm.c:2458: WARNING: Definition list ends without a blank line; unexpected unindent. [docutils]
Documentation/gpu/drm-mm:506: ./drivers/gpu/drm/drm_gpuvm.c:2459: WARNING: Definition list ends without a blank line; unexpected unindent. [docutils]

Fix these by wrapping drm_gpuvm_sm_map_exec_lock() expected usage
example in literal code block.

Fixes: 471920c ("drm/gpuvm: Add locking helpers")
Reported-by: Stephen Rothwell <[email protected]>
Closes: https://lore.kernel.org/linux-next/[email protected]/
Signed-off-by: Bagas Sanjaya <[email protected]>
Acked-by: Randy Dunlap <[email protected]>
Tested-by: Randy Dunlap <[email protected]>
Acked-by: Danilo Krummrich <[email protected]>
Patchwork: https://patchwork.freedesktop.org/patch/663121/
Signed-off-by: Rob Clark <[email protected]>

Author: bagasme

drivers/gpu/drm/drm_gpuvm.c

@@ -2430,7 +2430,7 @@ static const struct drm_gpuvm_ops lock_ops = {
  * remapped, and locks+prepares (drm_exec_prepare_object()) objects that
  * will be newly mapped.
  *
- * The expected usage is:
+ * The expected usage is::
  *
  *    vm_bind {
  *        struct drm_exec exec;