drm/i915/gvt: use hardcoded reference clocks

drm/i915/gvt: use hardcoded reference clocks

Usually I'd argue hardcoding values is the wrong thing to do, but in
this case, GVT looking deep into the guts of the DPLL manager for the
reference clocks is worse. This is done for BDW and BXT only, and there
shouldn't be any reason to try to be so dynamic about it.

This helps reduce the direct pokes at display guts from non-display
code.

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Zhenyu Wang <zhenyuw.linux@gmail.com>
Link: https://lore.kernel.org/r/20250321125114.750062-1-jani.nikula@intel.com
Signed-off-by: Jani Nikula <jani.nikula@intel.com>