drm: Add directive to format code in comment

Add formatting directive line in function `drm_gpuvm_sm_map_exec_lock()`
comment to clear warning messages shown bellow that appears generating
documentation `make htmldocs`.

  Warning: ./drivers/gpu/drm/drm_gpuvm.c:2444: Unexpected indentation.
  Warning: ./drivers/gpu/drm/drm_gpuvm.c:2446: Block quote ends without a blank line; unexpected unindent.
  Warning: ./drivers/gpu/drm/drm_gpuvm.c:2450: Definition list ends without a blank line; unexpected unindent.
  Warning: ./drivers/gpu/drm/drm_gpuvm.c:2451: Definition list ends without a blank line; unexpected unindent.
  Warning: ./drivers/gpu/drm/drm_gpuvm.c:2455: Unexpected indentation.
  Warning: ./drivers/gpu/drm/drm_gpuvm.c:2456: Definition list ends without a blank line; unexpected unindent.
  Warning: ./drivers/gpu/drm/drm_gpuvm.c:2457: Definition list ends without a blank line; unexpected unindent.
  Warning: ./drivers/gpu/drm/drm_gpuvm.c:2458: Definition list ends without a blank line; unexpected unindent.

Fixes: 471920c ("drm/gpuvm: Add locking helpers")
Signed-off-by: Javier Garcia <rampxxxx@gmail.com>
Reviewed-by: Bagas Sanjaya <bagasdotme@gmail.com>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@oss.qualcomm.com>
Link: https://lore.kernel.org/r/20250810150706.305040-1-rampxxxx@gmail.com
[ Fix typo in commit message. - Danilo ]
Signed-off-by: Danilo Krummrich <dakr@kernel.org>

Author: rampxxxx

drivers/gpu/drm/drm_gpuvm.c

@@ -2432,6 +2432,8 @@ static const struct drm_gpuvm_ops lock_ops = {
  *
  * The expected usage is:
  *
+ * .. code-block:: c
+ *
  *    vm_bind {
  *        struct drm_exec exec;
  *