drm/amdgpu: move default gart size setting into gmc modules

Move the asic specific code into the IP modules.

Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>