msm: mdss: enable VBIF write_gather for WB2

LM provides data stream with some write gaps that are propagated
through VBIF to AXI. If write gap is big enough it hangs AXI/BMIC.
In order to ensure there are no gaps in the outgoing data transfer
to AXI interface, all write data must available before initiating
an access. Starting with SDE3xx, the outgoing burst data collection
function is implemented in VBIF module, and it is HW recommendation
to enable this for WB2.

Change-Id: I6f0495068dd6344f8cd161175947391e5998b8d4
Signed-off-by: Veera Sundaram Sankaran <veeras@codeaurora.org>