drm/radeon: narrow scope of Apple re-POST hack

This narrows the scope of the apple re-POST hack added in:
drm/radeon: re-POST the asic on Apple hardware when booted via EFI

That patch prevents UVD from working on macs when booted in EFI
mode.  The original patch fixed macbook2,1 systems which were
r5xx and hence have no UVD.  Limit the hack to those systems to
prevent UVD breakage on newer systems.

Fixes:
https://bugs.freedesktop.org/show_bug.cgi?id=63935

Cc: Matthew Garrett <mjg59@srcf.ucam.org>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Acked-by: Matthew Garrett <matthew.garrett@nebula.com>