PCI: mediatek: Remove the redundant dev->pm_domain check

There is no need to check whether device have a PM domain attached before
calling the PM runtime methods. Remove it.

Signed-off-by: Honghui Zhang <honghui.zhang@mediatek.com>
[lorenzo.pieralisi@arm.com: commit log changes]
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Acked-by: Ryder Lee <ryder.lee@mediatek.com>