msm: mdss: Distinguish BL_DCS_CMD from DCS_CMD on 8x10

BL_DCS_CMD should be sent in HS mode on 8x10 as that could
reduce the dcs command transfer time in blank period.

Change-Id: I841dc788808ea10848f0b06d5b3f4fa010f51b17
Signed-off-by: Shuo Yan <shuoy@codeaurora.org>