PCI: keystone: Pass keystone_pcie, not va_app_base, to DBI functions

Instead of passing ks_pcie->va_app_base to DBI mode functions,
pass the struct keystone_pcie.  This will allow them to use register
accessors.  No functional change intended.

Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>