drm/msm/dsi-staging: allow empty timing for DSI external mode

If external DSI bridge has implemented connector functions, we
allow empty timing information in DSI panel device tree file.
In this mode, all the timing information comes from external
bridge, e.g. from EDID, and DSI controller will be configured
accordingly.

If timing nodes are defined in DSI panel, they will overwrite
the modes from external bridge. This mode is useful when no EDID
or wrong EDID are reported, or dedicated mode is selected.

CRs-Fixed: 2268280
Change-Id: I6a929d7f3f7a6bbeed04f61a317c5c07f4643aa8
Signed-off-by: Xiaowen Wu <wxiaowen@codeaurora.org>