board: rockchip: Refactor panel auto-detect code

Make the inability to detect a panel using the auto detection code not
fail the entire boot process. This means that if the panel ID cannot
be read we don't set an environment variable for the panel, and if an
environment variable for the panel is not set we don't attempt to
update the compatible string. Changes to the code also ensure that
when there are multiple compatible strings required for the panel
we use them both, which solves some issues that will pop up soon
for the Linux driver.

Signed-off-by: Chris Morgan <macromorgan@hotmail.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>