drm/amdgpu: Prefer shadow rom when available

Fetch VBIOS from shadow ROM when available before trying other methods
like EFI method.

Signed-off-by: Lijo Lazar <lijo.lazar@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Fixes: 9c081c11 ("drm/amdgpu: Reorder to read EFI exported ROM first")
Closes: drm/amd#4066


Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org