drm/radeon: Ignore RADEON_GEM_GTT_WC on 32-bit x86

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=84627


Signed-off-by: Michel Dänzer <michel.daenzer@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org