drm/radeon: implement ACPI VFCT vbios fetch (v3)

commit 268ba0a99f89a84dc5eb312470896113d0709c74 upstream.

This is required for pure UEFI systems.  The vbios is stored
in ACPI rather than at the legacy vga location.

Fixes:
https://bugs.freedesktop.org/show_bug.cgi?id=26891

V2: fix #ifdefs as per Greg's comments
V3: fix it harder

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Jerome Glisse <jglisse@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>