drm/msm/mdp4: Fix issue with LCDC/LVDS port parsing

The LVDS port is the first in the list of the output ports in MDP4.
The driver assumed that if the port and its corresponding endpoint
is defined, then there should be a panel node too. This isn't
necessary since boards may not really use a LVDS panel. Don't fail
if there isn't a panel node available.

While we're at it, use of_graph_get_endpoint_by_regs instead of
of_graph_get_next_endpoint to make it more explicit that the LVDS
output is at port 0.

Tested-by: John Stultz <john.stultz@linaro.org>
Signed-off-by: Archit Taneja <architt@codeaurora.org>
Signed-off-by: Rob Clark <robdclark@gmail.com>