drivers/pcie: Add a function to know MSI/MSI-x support of an endpoint

And since it does yet another round of pcie_get_cap() on PCIE_MSI_CAP_ID
and PCIE_MSIX_CAP_ID, let's factorize that into a utility function and
change the relevant places to use that function instead.

Signed-off-by: Tomasz Bursztyka <tomasz.bursztyka@linux.intel.com>