drm/doc: fix duplicate declaration warning

We have duplicate kernel-doc directives for the same struct, leading to:

/home/jani/src/linux/Documentation/gpu/driver-uapi.rst:2279: WARNING: Duplicate C declaration, also defined at rfc/i915_scheduler:3.
Declaration is '.. c:struct:: i915_context_engines_parallel_submit'.

Use the Sphinx C domain namespace for the rfc document to fix this.
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Luca Coelho <luciano.coelho@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230621123156.14907-1-jani.nikula@intel.com

Documentation/gpu/rfc/i915_scheduler.rst

@@ -135,9 +135,13 @@ Add I915_CONTEXT_ENGINES_EXT_PARALLEL_SUBMIT and
 drm_i915_context_engines_parallel_submit to the uAPI to implement this
 extension.
 
+.. c:namespace-push:: rfc
+
 .. kernel-doc:: include/uapi/drm/i915_drm.h
         :functions: i915_context_engines_parallel_submit
 
+.. c:namespace-pop::
+
 Extend execbuf2 IOCTL to support submitting N BBs in a single IOCTL
 -------------------------------------------------------------------
 Contexts that have been configured with the 'set_parallel' extension can only