drm/amdgpu: Remove checking for atombios

This is a left over from radeon, amdgpu doesn't support any
non-atombios parts and amdgpu_device_init would bail if the
check for atombios failed anyway.

Reviewed-by: Edward O'Callaghan <funfunctor@folklore1984.net>
Signed-off-by: Nils Wallménius <nils.wallmenius@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>