drm/i915/mtl: Remove misleading "clock" field from C20 pll_state

The field link_bit_rate serves as the actual clock value for the C20
pll_state structure. Remove the misleading clock field. The subsequent
patch would rename the link_bit_rate as the 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/20231207221025.2032207-3-radhakrishna.sripada@intel.com