msm: mdss: configure DSC pkt_per_line correctly

DSC pkt_per_line is not mapping to register directly.
value 0 is for 1 pkt, value 1 for 2 pkt and value 2
is for 4 pkt.  3 pkt per line is not support.

Change-Id: I43ff34018668feb7a52eaa8f4b2de632fc9ab59b
Signed-off-by: Kuogee Hsieh <khsieh@codeaurora.org>
Signed-off-by: Ujwal Patel <ujwalp@codeaurora.org>