drm/mediatek: Fix backport issue in mtk_drm_gem_prime_vmap()

When building with clang:

  drivers/gpu/drm/mediatek/mtk_drm_gem.c:255:10: error: incompatible integer to pointer conversion returning 'int' from a function with result type 'void *' [-Wint-conversion]
    255 |                 return -ENOMEM;
        |                        ^~~~~~~
  1 error generated.

GCC reports the same issue as a warning, rather than an error.

Prior to commit 7e542ff8b463 ("drm/mediatek: Use struct dma_buf_map in
GEM vmap ops"), this function returned a pointer rather than an integer.
This function is indirectly called in drm_gem_vmap(), which treats NULL
as -ENOMEM through an error pointer. Return NULL in this block to
resolve the warning but keep the same end result.

Fixes: 43f561e809aa ("drm/mediatek: Fix potential memory leak if vmap() fail")
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/gpu/drm/mediatek/mtk_drm_gem.c

@@ -252,7 +252,7 @@ void *mtk_drm_gem_prime_vmap(struct drm_gem_object *obj)
 	if (!mtk_gem->kvaddr) {
 		kfree(sgt);
 		kfree(mtk_gem->pages);
-		return -ENOMEM;
+		return NULL;
 	}
 out:
 	kfree(sgt);