drm/amdgpu: stop using BO status for user pages

Instead use a counter to figure out if we need to set new pages or not.

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