drm/amdgpu: add automatic per asic settings for gart_size

We need a larger gart for asics that do not support GPUVM on all
engines (e.g., MM) to make sure we have enough space for all
gtt buffers in physical mode.  Change the default size based on
the asic type.

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