video: omap: split the legacy code from the DM code

The schedule for deprecating the features of the pre-driver-model puts
2019.17 as the deadline for the video subsystem. Furthermore, the latest
patches applied to the am335x-fb.c module have decreased the amount of
code shared with the pre-driver-model implementation. Splitting the two
implementations into two modules improves the readability of the code
and will make it easier to drop the pre-driver-model code.
I have not created a header file with the data structures and the
constants for accessing the LCD controller registers, but I preferred to
keep them inside the two c modules. This is a code replication until the
pre-driver-model version is dropped.

Signed-off-by: Dario Binacchi <dariobin@libero.it>