drm/amdgpu: add function parameter description in 'amdgpu_gart_bind'

Fixes gcc warning:

drivers/gpu/drm/amd/amdgpu/amdgpu_gart.c:313: warning: Function
parameter or member 'flags' not described in 'amdgpu_gart_bind'

Fixes: d38ceaf9 ("drm/amdgpu: add core driver (v4)")
Signed-off-by: yu kuai <yukuai3@huawei.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>