drm/amdgpu: init return value in amdgpu_ttm_clear_buffer

drm/amdgpu: init return value in amdgpu_ttm_clear_buffer

Otherwise an uninitialized value can be returned if
amdgpu_res_cleared returns true for all regions.

Possibly closes: https://gitlab.freedesktop.org/drm/amd/-/issues/3812

Fixes: a68c7eaa7a8f ("drm/amdgpu: Enable clear page functionality")
Signed-off-by: Pierre-Eric Pelloux-Prayer <pierre-eric.pelloux-prayer@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>