radeon: use pdev->pm_cap instead of pci_find_capability(..,PCI_CAP_ID_PM)

Pci core has been saved pm cap register offset by pdev->pm_cap in pci_pm_init()
in init path. So we can use pdev->pm_cap instead of using
pci_find_capability(pdev, PCI_CAP_ID_PM) for better performance and simplified code.

Signed-off-by: Yijing Wang <wangyijing@huawei.com>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Jean-Christophe Plagniol-Villard <plagnioj@jcrosoft.com>
Cc: Tomi Valkeinen <tomi.valkeinen@ti.com>
Cc: linux-fbdev@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>