drm/amdgpu: Remove superfluous void * cast in debugfs_create_file() call

There is no need to cast a typed pointer to a void pointer when calling
a function that accepts the latter.  Remove it, as the cast prevents
further compiler checks.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>