drm/amdgpu: immediately use GTT for new allocations

Only use GTT as a fallback if we already have a backing store. This
prevents evictions when an application constantly allocates and frees new
memory.

Partially fixes
https://gitlab.freedesktop.org/drm/amd/-/issues/3844#note_2833985

.

Signed-off-by: Christian König <christian.koenig@amd.com>
Fixes: 216c1282 ("drm/amdgpu: use GTT only as fallback for VRAM|GTT")
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org