disp: msm: dsi: allocate DSI command buffer during bind

The DMA buffer allocation for DSI happens during the first
command transfer. This change moves this allocation to happen during
bind.

Change-Id: I7969a019a8b84282e8a153f5393c9a3de5a28043
Signed-off-by: Satya Rama Aditya Pinapala <psraditya30@codeaurora.org>