drm/mm: add ig_frag selftest

This patch introduces fragmentation in the address range
and measures time taken by 10k and 20k insertions. ig_frag()
will fail if the time taken by 20k insertions takes more than
4 times of 10k insertions as we know that insertions should at
most scale quadratically.

v2:
introduce fragmentation by freeing every other node.
only test bottom-up and top-down for now.

v3:
fix incorrect mode check

Signed-off-by: Nirmoy Das <nirmoy.das@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Christian König <christian.koenig@amd.com>
Link: https://patchwork.freedesktop.org/patch/369076/