drm/i915: Clean-up outdated struct_mutex comments

drm/i915: Clean-up outdated struct_mutex comments

The struct_mutex will be removed from the DRM subsystem, as it was a
legacy BKL that was only used by i915 driver. After review, it was
concluded that its usage was no longer necessary

This patch updates various comments in the i915 codebase to
either remove or clarify references to struct_mutex, in order to
prevent future misunderstandings.

* i915_drv.h: Removed the statement that stolen_lock is the inner lock
  when overlaps with struct_mutex, since struct_mutex is no longer used
  in the driver.
* i915_gem.c: Removed parentheses suggesting usage of struct_mutex, which
  which is no longer used.

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/20250908131518.36625-8-luiz.mello@estudante.ufscar.br
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>