freedreno/drm: Add sub-allocator

freedreno/drm: Add sub-allocator

Add a heap that we can use for allocations of small mappable buffers.
This avoids the churn of mmap/unmap, which is especially expensive in
a VM.  It also allows packing more smaller allocations together in a
page, which is useful for PIPE_BUFFERs (which are also mappable).

This avoid jank caused by the overhead of setting up or tearing down
guest mappings when running in a VM.  And also significantly reduces
the # of BOs referenced on a submit.

Signed-off-by: Rob Clark <robdclark@chromium.org>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/20263>