drm/amdgpu: check if vram is lost v2

backup first 64 byte of gart table as reset magic, check if magic is same
after gpu hw reset.
v2: use memcmp instead of manual innovation.

Signed-off-by: Chunming Zhou <David1.Zhou@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>