drm/panthor: Remove dead VM flushing code

Commit ec62d37d("drm/panthor: Fix the fast-reset logic") did away
with the only reference to panthor_vm_flush_all(), so let's get rid
of the orphaned definition.

Signed-off-by: Adrián Larumbe <adrian.larumbe@collabora.com>
Reviewed-by: Liviu Dudau <liviu.dudau@arm.com>
Reviewed-by: Steven Price <steven.price@arm.com>
Signed-off-by: Steven Price <steven.price@arm.com>
Link: https://lore.kernel.org/r/20250711154557.739326-1-adrian.larumbe@collabora.com