drm/amdgpu/display: handle multiple numbers of fclks in dcn_calcs.c (v2)

We might get different numbers of clocks from powerplay depending
on what the OEM has populated.

v2: add assert for at least one level

Bug: https://gitlab.freedesktop.org/drm/amd/issues/963
Reviewed-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>