drm/omap: fix dmabuf mmap for dma_alloc'ed buffers

omap_gem_dmabuf_mmap() returns an error (with a WARN) when called for a
buffer which is allocated with dma_alloc_*(). This prevents dmabuf mmap
from working on SoCs without DMM, e.g. AM4 and OMAP3.

I could not find any reason for omap_gem_dmabuf_mmap() rejecting such
buffers, and just removing the if() fixes the limitation.
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>

drivers/gpu/drm/omapdrm/omap_gem_dmabuf.c

@@ -147,9 +147,6 @@ static int omap_gem_dmabuf_mmap(struct dma_buf *buffer,
 	struct drm_gem_object *obj = buffer->priv;
 	int ret = 0;
 
-	if (WARN_ON(!obj->filp))
-		return -EINVAL;
-
 	ret = drm_gem_mmap_obj(obj, omap_gem_mmap_size(obj), vma);
 	if (ret < 0)
 		return ret;