drm/msm/sde: enable write_gather_en in VBIF for all write clients

Setting the write_gather_en, ensures there are no gaps in
the outstanding data transfer to AXI interface. Starting
SDE3xx, the outgoing burst data collection function is
implemented in VBIF module, and the Hardware Programming
mandates enabling the write_gather_en bit.

Change-Id: I675f377129621950433929e4c221fb94399ef8b5
Signed-off-by: Veera Sundaram Sankaran <veeras@codeaurora.org>