drm/xe/pci: Introduce a helper to allow VF access to PF xe_device

drm/xe/pci: Introduce a helper to allow VF access to PF xe_device

In certain scenarios (such as VF migration), VF driver needs to interact
with PF driver.
Add a helper to allow VF driver access to PF xe_device.

Reviewed-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Link: https://patch.msgid.link/20251127093934.1462188-3-michal.winiarski@intel.com
Signed-off-by: Michał Winiarski <michal.winiarski@intel.com>
(cherry picked from commit 8b3cce3ad9c78ce3dae1c178f99352d50e12a3c0)
Signed-off-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>