drm/radeon: update comments to clarify VM setup (v2)

The actual set up and assignment of VM page tables
is done on the fly in radeon_gart.c.

v2: update vm size comments

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