drm/panfrost: Check sgt to know whether pages are already mapped

drm/panfrost: Check sgt to know whether pages are already mapped

In the MMU's page fault ISR for a heap object, determine whether the
faulting address belongs to a 2MiB block that was already mapped by
checking its corresponding sgt in the Panfrost BO.

This is done in preparation for a future commit in which the MMU mapping
helper might fail, but the page array is left populated, so this cannot
be used as a check for an early bail-out.

Reviewed-by: Steven Price <steven.price@arm.com>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Signed-off-by: Adrián Larumbe <adrian.larumbe@collabora.com>
Link: https://lore.kernel.org/r/20251019145225.3621989-6-adrian.larumbe@collabora.com
Signed-off-by: Steven Price <steven.price@arm.com>