drm/amdgpu: consistent name all GART related parts

Rename symbols from gtt_ to gart_ as appropriate.

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