Revert "drm/amd/amdgpu: set gtt size according to system memory size only"

This reverts commit ba851eed895c76be0eb4260bdbeb7e26f9ccfaa2.
With that change piglit max size tests (running with -t max.*size) are causing
OOM and hard hang on my CZ with 1GB RAM.

Signed-off-by: Andrey Grodzovsky <andrey.grodzovsky@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Roger He <Hongbo.He@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>