drm/amdgpu: Refine function name

change function name gfx_v6/7/8/9_0_gpu_init to
gfx_v6/7/8/9_0_constants_init.
this function is just for init gfx constants such
as max pipes, render backends...

Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Rex Zhu <Rex.Zhu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>