drm/amdgpu: fix to miss program invalidation at resume

This patch moves invalidation into gart enable function from hw_init.
Because we would like align the sequence calling between init and resume.

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