drm/amd/display: add sanity check for clk table from smu

[Why]
Handle the case where we don't get a valid table. Also fixes compiler
warning for variable potentially used before assignment.

[How]
If the entire table has no valid fclk, reject the table and use our own
hard code.

Signed-off-by: Eric Yang <Eric.Yang2@amd.com>
Acked-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Reviewed-by: Roman Li <Roman.Li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>