drm/amdgpu: remove unnecessary check for mem train

a.Check whether mem train support when try to reserve related memory.
b.Remove ASIC check and atom firmware table version check as the check
of firmware capability is enough to achieve that purpose.

Signed-off-by: Likun Gao <Likun.Gao@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>