drm/xe/vf: Don't expose sysfs attributes not applicable for VFs

VFs can't read BMG_PCIE_CAP(0x138340) register nor access PCODE
(already guarded by the info.skip_pcode flag) so we shouldn't
expose attributes that require any of them to avoid errors like:

 [] xe 0000:03:00.1: [drm] Tile0: GT0: VF is trying to read an \
                     inaccessible register 0x138340+0x0
 [] RIP: 0010:xe_gt_sriov_vf_read32+0x6c2/0x9a0 [xe]
 [] Call Trace:
 []  xe_mmio_read32+0x110/0x280 [xe]
 []  auto_link_downgrade_capable_show+0x2e/0x70 [xe]
 []  dev_attr_show+0x1a/0x70
 []  sysfs_kf_seq_show+0xaa/0x120
 []  kernfs_seq_show+0x41/0x60

Fixes: 0e414bf7 ("drm/xe: Expose PCIe link downgrade attributes")
Fixes: cdc36b66 ("drm/xe: Expose fan control and voltage regulator version")
Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Cc: Lucas De Marchi <lucas.demarchi@intel.com>
Cc: Lukasz Laguna <lukasz.laguna@intel.com>
Reviewed-by: Raag Jadav <raag.jadav@intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://lore.kernel.org/r/20250916170029.3313-2-michal.wajdeczko@intel.com