drm/i915/display: Remove outdated struct_mutex comments

drm/i915/display: Remove outdated struct_mutex comments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 update a comment about struct_mutex in i915/display, in order
to prevent future misunderstandings.

* intel_fbc.c: Removed the statement that intel_fbc->lock is the inner
  lock when overlapping with struct_mutex, since struct_mutex is no
  longer used anywhere in the driver.

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/20250908131518.36625-7-luiz.mello@estudante.ufscar.br
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>