drm/amdgpu/bios: split vbios fetching between APU and dGPU

drm/amdgpu/bios: split vbios fetching between APU and dGPU

We need some different logic for dGPUs and the APU path
can be simplified because there are some methods which
are never used on APUs.  This also fixes a regression
on some older APUs causing the driver to fetch the
unpatched ROM image rather than the patched image.

Fixes: 9c081c11c621 ("drm/amdgpu: Reorder to read EFI exported ROM first")
Reviewed-by: George Zhang <George.zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>