drm/i915: cache the results in vlv_clock_get_hpll_vco() and use it more

drm/i915: cache the results in vlv_clock_get_hpll_vco() and use it more

Use vlv_clock_get_hpll_vco() helper more to avoid looking at
i915->hpll_freq directly. Cache and return the cached results to avoid
repeated lookups.

v2: Rebase

Reviewed-by: Michał Grzelak <michal.grzelak@intel.com> # v1
Acked-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://lore.kernel.org/r/14695618682d8d8fad1adc485de7a122c8e1494a.1757688216.git.jani.nikula@intel.com
Signed-off-by: Jani Nikula <jani.nikula@intel.com>