drm/radeon: allow creating overlapping userptrs

Similar to the Intel implementation, but instead of just falling back to a
global linear list when we have an overlapping userptr request we accumulate
all overlapping userptrs in a local list.

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