drm/amd/display: use requested_dispclk_khz instead of clk

Use requested_dispclk_khz / 1000 directly

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