staging: omapdrm: Fix DMM sparse warnings

Fix the following sparse warnings:

drivers/staging/omapdrm/omap_dmm_tiler.c:123:13:
   warning: symbol 'omap_dmm_irq_handler' was not declared.
   Should it be static?

drivers/staging/omapdrm/omap_dmm_tiler.c:370:24:
   warning: Using plain integer as NULL pointer

Signed-off-by: Andy Gross <andy.gross@ti.com>
Signed-off-by: Rob Clark <rob.clark@linaro.org>
Reviewed-by: Sumit Semwal <sumit.semwal@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>