drm/amdgpu: Add helper to alloc GART entries

drm/amdgpu: Add helper to alloc GART entries

Add helper amdgpu_gtt_mgr_alloc/free_entries, define
GART_ENTRY_WITHOUT_BO_COLOR color for GART node not allocated with GTT
bo, then amdgpu_gtt_mgr_recover skip those mm_node.

Signed-off-by: Philip Yang <Philip.Yang@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Pierre-Eric Pelloux-Prayer <pierre-eric.pelloux-prayer@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>