drm/amdgpu: add vangogh_reg_base_init function for van gogh

This patch adds vangogh_reg_base_init function to init the register base for
van gogh.

v2: make vangogh_reg_base_init void, align equality sign

Signed-off-by: Huang Rui <ray.huang@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>