mdss: display: send multiple dcs commands with one transmission

Packing consecutive dcs commands into one single buffer if their
dcs header's "last" flag byte is not set. The packing process will
stop after packing first dcs command with "last" flag byte set.
Therefore multiple dcs commands can be send to panel with one
transmission.

CRs-Fixed: 486748
Change-Id: I76d2a043788638579a62a7f949309f049fc999d7
Signed-off-by: Kuogee Hsieh <khsieh@codeaurora.org>