drm/amdgpu: remove GTT accounting v2

This is provided by TTM now.

Also switch man->size to bytes instead of pages and fix the double
printing of size and usage in debugfs.

v2: fix size checking as well

Signed-off-by: Christian König <christian.koenig@amd.com>
Tested-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220214093439.2989-6-christian.koenig@amd.com