drm/doc: fix duplicate declaration warning

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