drm/panthor: Don't update MMU_INT_MASK in panthor_mmu_irq_handler()

drm/panthor: Don't update MMU_INT_MASK in panthor_mmu_irq_handler()

Interrupts are automatically unmasked in
panthor_mmu_irq_threaded_handler() when the handler returns. Unmasking
prematurely might generate spurious interrupts if the IRQ line is
shared.

Changes in v2:
- New patch

Changes in v3:
- Add R-bs

Reviewed-by: Liviu Dudau <liviu.dudau@arm.com>
Reviewed-by: Steven Price <steven.price@arm.com>
Link: https://lore.kernel.org/r/20250404080933.2912674-6-boris.brezillon@collabora.com
Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>