drm/tests: Add a unit test for range bias allocation

Allocate cleared blocks in the bias range when the DRM
buddy's clear avail is zero. This will validate the bias
range allocation in scenarios like system boot when no
cleared blocks are available and exercise the fallback
path too. The resulting blocks should always be dirty.

v1:(Matthew)
  - move the size to the variable declaration section.
  - move the mm.clear_avail init to allocator init.

Signed-off-by: Arunpravin Paneer Selvam <Arunpravin.PaneerSelvam@amd.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240514145636.16253-2-Arunpravin.PaneerSelvam@amd.com