drm/radeon: track pinned memory (v2)

So we know how large an allocation we can allow.

v2: incorporate Michel's comments

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