drm/amdgpu: check number of gfx ring before init cp gfx

Check number of gfx ring, rather than asic type,
before cp gfx engine initialization so driver just
need to make sure number of gfx ring is initialized
correctly in gfx early_init phase. No need to add
additional asic type check everywhere when there is
new asic with gfx pipe removed.

Signed-off-by: Hawking Zhang <Hawking.Zhang@amd.com>
Reviewed-by: Feifei Xu <Feifei.Xu@amd.com>
Acked-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>