disp: msm: fix dsi debugbus in-mem logging

Update the dump_mem pointer offset while storing the debugbus
data for the second DSI to avoid overwriting to same memory.
As part of the change, register the DSI ctrl with sde_dbg from
ctrl_init directly, instead of debugfs_init to avoid code replication.

Change-Id: I4089f3038ffa89136eaea956d27270f638a99043
Signed-off-by: Veera Sundaram Sankaran <veeras@codeaurora.org>
Signed-off-by: Andhavarapu Karthik <kartkart@codeaurora.org>