drm/amdgpu/display: fix warning on 32 bit in dmub

Use uintptr_t rather than uint64_t.

Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: Dave Airlie <airlied@gmail.com>
Cc: Harry Wentland <Harry.Wentland@amd.com>