drm/amdgpu/discovery: move common discovery code out of navi1*_reg_base_init()

move amdgpu_discovery_reg_base_init() from navi1*_reg_base_init() to a
common function nv_reg_base_init().

Signed-off-by: Xiaojie Yuan <xiaojie.yuan@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>