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

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: 471920ce25d5 ("drm/gpuvm: Add locking helpers")
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Closes: https://lore.kernel.org/linux-next/20250708192038.6b0fd31d@canb.auug.org.au/
Signed-off-by: Bagas Sanjaya <bagasdotme@gmail.com>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Tested-by: Randy Dunlap <rdunlap@infradead.org>
Acked-by: Danilo Krummrich <dakr@kernel.org>
Patchwork: https://patchwork.freedesktop.org/patch/663121/
Signed-off-by: Rob Clark <robin.clark@oss.qualcomm.com>