OMAPDSS: Move definition of DEBUG flag to Makefile

In OMAPDSS the DEBUG flag is set only after the OMAPDSS module is called, for
which the debugging capabilities are available only after its proper
initialization. As a result of which tracking of bugs prior to or during initial
process becomes difficult. So, the definition of DEBUG is being moved to the
corresponding Makefile.

Signed-off-by: Chandrabhanu Mahapatra <cmahapatra@ti.com>
Reviewed-by: Sumit Semwal <sumit.semwal@ti.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>