drm/amdgpu: move reg base init forward for soc15

Init soc15 reg base early enough so we can touch
mailbox related registers in request full access
for sriov before set_ip_blocks, vi&nv arch doesn't
use reg base in virt ops.

v2: fix reg_base_init missed in bare metal case.

Signed-off-by: Wenhui Sheng <Wenhui.Sheng@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>