disp: msm: dsi: update CPHY command mode clock calculation

In CPHY, packet header and checksum is sent twice and SYNC is
sent in between two headers. So, increase packet overhead used
in clock calculation to 15 bytes. Packet Header: 8 bytes,
CRC: 4 bytes, SYNC: 2 bytes and dcs command: 1 byte.

Change-Id: I7a1160cbb57ba4f1faeb4b36a16c322e6069d58f
Signed-off-by: Ritesh Kumar <riteshk@codeaurora.org>