vout: add dummy_panel display mode [1/1]

vout: add dummy_panel display mode [1/1]

PD#SWPL-5017

Problem:
need support dummy lcd for viu1

Solution:
add dummp_panel display mode.
when need viu1 + viu2 -> encl, and show viu2 data,
you can use below setting:
echo dump_panel >/sys/class/display/mode
echo panel >/sys/class/display2/mode

Verify:
x301

Change-Id: I8a7fb52e2272fc8f71d682424ff20dc37f02b726
Signed-off-by: Evoke Zhang <evoke.zhang@amlogic.com>