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>

drivers/staging/omapdrm/omap_dmm_tiler.c

@@ -120,7 +120,7 @@ static int wait_status(struct refill_engine *engine, uint32_t wait_mask)
 	return 0;
 }
 
-irqreturn_t omap_dmm_irq_handler(int irq, void *arg)
+static irqreturn_t omap_dmm_irq_handler(int irq, void *arg)
 {
 	struct dmm *dmm = arg;
 	uint32_t status = readl(dmm->base + DMM_PAT_IRQSTATUS);
@@ -367,7 +367,7 @@ struct tiler_block *tiler_reserve_1d(size_t size)
 	int num_pages = (size + PAGE_SIZE - 1) >> PAGE_SHIFT;
 
 	if (!block)
-		return 0;
+		return ERR_PTR(-ENOMEM);
 
 	block->fmt = TILFMT_PAGE;