drm/radeon: Fix VRAM size calculation for VRAM >= 4GB

commit fc986034540102cd090237bf3f70262e1ae80d9c upstream.

Add ULL prefix to avoid overflow.

Signed-off-by: Niels Ole Salscheider <niels_ole@salscheider-online.de>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>