Documentation/gpu: fix Panfrost documentation build warnings

Documentation/gpu: fix Panfrost documentation build warnings

Fix issues revealed by `make htmldocs` after adding Panfrost DRM
documentation file.

Signed-off-by: Adrián Larumbe <adrian.larumbe@collabora.com>
Fixes: f11b0417eec2 ("drm/panfrost: Add fdinfo support GPU load metrics")
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202310030917.Txzlpoeq-lkp@intel.com
Reviewed-by: Steven Price <steven.price@arm.com>
Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231005141239.132783-1-adrian.larumbe@collabora.com