drm/i915: Remove struct_mutex in i915_irq.c

drm/i915: Remove struct_mutex in i915_irq.c

Remove struct_mutex from ivb_parity_work() function.

The ivb_parity_work runs in a workqueue so it cannot race with itself.

Also, it is not protecting anything with the other remaining usage of
struct_mutex.

Signed-off-by: Luiz Otavio Mello <luiz.mello@estudante.ufscar.br>
Suggested-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://lore.kernel.org/r/20250908131518.36625-3-luiz.mello@estudante.ufscar.br
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>