drm/i915/gvt: get msi cap offset from pdev directly

Linux PCI driver saves the msi and msix capability offset
in pci_dev->msi_cap and pci_dev->msix_cap. We can use msi_cap
in pci_dev directly, no need hardcode.

Signed-off-by: Du, Changbin <changbin.du@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>