msm: mdss: fix the RGB666 PACK_ALIGN setting for dsi

For RGB 666 panel format on DSI, a change has been made earlier to set the
alignment to MSB. This was causing color corruption on EDP panel as it
requires LSB alignment. This fix is to set the MSB alignment for RGB666
format only on DSI panels, so that it doesn't affect EDP.

Change-Id: If07b915bb30b00ff1824c7c662bbf4db8a217a6b
Signed-off-by: Vineet Bajaj <vbajaj@codeaurora.org>