PCI: Add a helper to check Power Resource Requirements _PR3 existence

PCI: Add a helper to check Power Resource Requirements _PR3 existence

[ Upstream commit 52525b7a3cf82adec5c6cf0ecbd23ff228badc94 ]

A driver may want to know the existence of _PR3, to choose different
runtime suspend behavior. A user will be add in next patch.

This is mostly the same as nouveau_pr3_present().

Signed-off-by: Kai-Heng Feng <kai.heng.feng@canonical.com>
Acked-by: Bjorn Helgaas <bhelgaas@google.com>
Link: https://lore.kernel.org/r/20191018073848.14590-1-kai.heng.feng@canonical.com
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Sasha Levin <sashal@kernel.org>