winsys/amdgpu: use 128KB BOs for suballocations of up to 64KB BOs

This decreases the number of BOs, but might also increase memory usage.
It's better for small textures.

The gameplay is on the far right:
https://people.freedesktop.org/~mareko/suballoc.svg

Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com>

src/gallium/winsys/amdgpu/drm/amdgpu_bo.c

@@ -502,8 +502,9 @@ struct pb_slab *amdgpu_bo_slab_alloc(void *priv, unsigned heap,
    if (!slab)
       return NULL;
 
+   unsigned slab_size = 1 << AMDGPU_SLAB_BO_SIZE_LOG2;
    slab->buffer = amdgpu_winsys_bo(amdgpu_bo_create(&ws->base,
-                                                    64 * 1024, 64 * 1024,
+                                                    slab_size, slab_size,
                                                     domains, flags));
    if (!slab->buffer)
       goto fail;

src/gallium/winsys/amdgpu/drm/amdgpu_winsys.h

@@ -41,8 +41,9 @@
 
 struct amdgpu_cs;
 
-#define AMDGPU_SLAB_MIN_SIZE_LOG2 9
-#define AMDGPU_SLAB_MAX_SIZE_LOG2 14
+#define AMDGPU_SLAB_MIN_SIZE_LOG2   9  /* 512 bytes */
+#define AMDGPU_SLAB_MAX_SIZE_LOG2   16 /* 64 KB */
+#define AMDGPU_SLAB_BO_SIZE_LOG2    17 /* 128 KB */
 
 struct amdgpu_winsys {
    struct radeon_winsys base;