drm: fix multi GPU drmGetDevices only return one device

When multi GPU present, after drmFoldDuplicatedDevices
merge same busid deveces, two different devices may be
seperated by zero in local_devices[]. The for loop
should check all local_devices instead of exit when
meet a zero.

Reviewed-by: Jim Qu <Jim.Qu@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Qiang Yu <Qiang.Yu@amd.com>