pci: msm: Refactor msm_pcie_confirm_linkup function

For link training Data link layer, active bit is checked
in link status register of the root complex. Hence create
a new function msm_pcie_dll_link_active() to check the link
status. Use same function in msm_pcie_confirm_linkup() as well.
In order to check endpoint access use pci_device_is_present()
provided by framework. This allows all ep config space access
to go through operation config callback from framework.
Also, remove reading of root complex vendor id and device id
which is redundant now.

Change-Id: I46fdb566ed20759d38d991ceeb23703759c04ecd
Signed-off-by: Hemant Kumar <hemantk@codeaurora.org>