drm/amdgpu: Bring bo creation in line with radeon driver (v2)

Add the bo creation changes that have been done to the radeon driver in
recent times, e.g. disable GTT WC on 32 bit because it is broken there,
and also disable it generally (and print a warning message) when
CONFIG_X86_PAT is not set.

v2: agd: fix warning in defined(CONFIG_X86) && !defined(CONFIG_X86_PAT)
    case

Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Nils Holland <nholland@tisys.org>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>