drm/radeon: use an intervall tree to manage the VMA v2

Scales much better than scanning the address range linearly.

v2: store pfn instead of address

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