drm/i915/mtl: Rename the link_bit_rate to clock in C20 pll_state

With the cleanup of the misleading clock value to avoid extra
calculations to convert between link_bit_rate and clock, use
one standard "clock" field for the c20 pll which works with
crtc_state->port_clock field.

Cc: Clint Taylor <clinton.a.taylor@intel.com>
Cc: Mika Kahola <mika.kahola@intel.com>
Signed-off-by: Radhakrishna Sripada <radhakrishna.sripada@intel.com>
Reviewed-by: Mika Kahola <mika.kahola@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231207221025.2032207-4-radhakrishna.sripada@intel.com