asahi: fix returning _Bool instead of pointer

When building for C23 the compiler warns about returning a boolean when
a different type is expected instead.

Change the code to return NULL instead of false, fixing the following
error:

-----------------------------------------------------------------------
../src/gallium/drivers/asahi/agx_pipe.c: In function ‘agx_resource_from_handle’:
../src/gallium/drivers/asahi/agx_pipe.c:184:14: error: incompatible types when returning type ‘_Bool’ but ‘struct pipe_resource *’ was expected
  184 |       return false;
      |              ^~~~~
-----------------------------------------------------------------------

Reviewed-by: Faith Ekstrand <faith.ekstrand@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/36323>

src/gallium/drivers/asahi/agx_pipe.c

@@ -181,7 +181,7 @@ agx_resource_from_handle(struct pipe_screen *pscreen,
     */
    if (rsc->modifier == DRM_FORMAT_MOD_LINEAR && (whandle->stride % 16) != 0) {
       FREE(rsc);
-      return false;
+      return NULL;
    }
 
    prsc = &rsc->base;