drm/i915: Remove todo and comments about struct_mutex

drm/i915: Remove todo and comments about struct_mutex

This patch completes the removal of struct_mutex from the driver.

Remove the related TODO item, as the transition away from struct_mutex
is now complete.

Also clean up references to struct_mutex in i915.rst to avoid outdated
documentation.

Signed-off-by: Luiz Otavio Mello <luiz.mello@estudante.ufscar.br>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://lore.kernel.org/r/20250908131518.36625-10-luiz.mello@estudante.ufscar.br
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>