Intel_dp->link_rate is not yet set at this point. Instead use
crtc_state->port_clock.

Fixes: 0dd21f83 ("drm/i915/psr: Silence period and lfps half cycle")
Signed-off-by: Jouni Högander <jouni.hogander@intel.com>
Reviewed-by: Animesh Manna <animesh.manna@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240409085759.178235-1-jouni.hogander@intel.com