drm/radeon: re-POST the asic on Apple hardware when booted via EFI

At least some Apples program the GPU into a state that wedges the engine
once userspace starts trying to perform accelerated operations. Executing
the Atom init scripts gets the hardware back into a working state. The
same hardware works fine when booted via BIOS emulation, so let's just
execute the init scripts on Apples when we're using EFI.

Signed-off-by: Matthew Garrett <mjg@redhat.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>