drm/radeon: add userptr flag to limit it to anonymous memory v2

Avoid problems with writeback by limiting userptr to anonymous memory.

v2: add commit and code comments

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

diff --cc drivers/gpu/drm/radeon/radeon_gem.c
Simple merge

diff --cc drivers/gpu/drm/radeon/radeon_ttm.c
Simple merge

diff --cc include/uapi/drm/radeon_drm.h
Simple merge