drm/amdgpu: switch gtt_mgr to counting used pages

Change mgr->available into mgr->used (invert the value).

Makes more sense to do it this way since we don't need the spinlock any
more to double check the handling.

v3 (chk): separated from the TEMPOARAY FLAG change.

Signed-off-by: Lang Yu <Lang.Yu@amd.com>
Signed-off-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Andrey Grodzovsky <andrey.grodzovsky@amd.com>
Acked-by: Nirmoy Das <nirmoy.das@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210622162339.761651-4-andrey.grodzovsky@amd.com