drm/xe/pm: Disable PM on unbounded pcie parent bridge

drm/xe/pm: Disable PM on unbounded pcie parent bridge

Intel Discrete GFX cards gfx may have multiple PCIe endpoints,
they connects to root port via pcie upstream switch port(USP)
and virtual pcie switch port(VSP), sometimes VSP pcie devices
doesn't bind to pcieport driver. Without pcieport driver, pcie PM
comes without any warranty and with unbounded VSP gfx card won't
transition to low power pcie Device and Link states therefore
assert drm_warn on unbounded VSP and disable xe driver
PM support.

v2:
- Disable Xe PCI PM support. [Rodrigo]
v3:
- Changed subject and Rebase.
v4:
- %s/xe_pci_unbounded_bridge_disable_pm/xe_assert_on_unbounded_bridge.
  [Rodrigo]
- Use device_set_pm_not_required() instead of dev_pm_ops NULL assignment.

Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Anshuman Gupta <anshuman.gupta@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230524090653.1192566-1-anshuman.gupta@intel.com
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>