OMAPDSS: Cleanup implementation of LCD channels

The current implementation of LCD channels and managers consists of a number of
if-else construct which has been replaced by a simpler interface. A constant
structure mgr_desc has been created in Display Controller (DISPC) module. The
mgr_desc contains for each channel its name, irqs and  is initialized one time
with all registers and their corresponding fields to be written to enable
various features of Display Subsystem. This structure is later used by various
functions of DISPC which simplifies the further implementation of LCD channels
and its corresponding managers.

Signed-off-by: Chandrabhanu Mahapatra <cmahapatra@ti.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>