drm/radeon: Always disable RADEON_GEM_GTT_UC along with RADEON_GEM_GTT_WC

Write-combining is a CPU feature. From the GPU POV, these both simply
mean no GPU<->CPU cache coherency.

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