drm/sde: add support for customized mode

Sometimes the HDMI is treated as non-pluggable display in auto
use cases. Add support to configure it through dtsi file, and
also provide timing parameters for the customized modes through
dtsi.

Change-Id: I2326b6c43cb7e6361be1f14d25f0e2e493c94177
Signed-off-by: Jin Li <jinl@codeaurora.org>