drm/panel/truly-r63350: Detect panel type from kernel cmdline

This is to support the use case that LK configures the panel for splash
screen and then passes the panel type to kernel via cmdline.  It's
a vendor bootloader/kernel specific implementation, and not suitable
for upstream.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>