drm/i915: Clean up gen2 DPLL readout

The current gen2 DPLL readout code:
* assumes i845/i865 have LVDS which is not true
* assumes only pipe B can drive LVDS (true, but makes
  the code appear a bit magical)
* hard to parse in general

Clean it up by checking for i85x (the only gen2 platform
with LVDS) and reusing intel_lvds_port_enabled().

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210715093530.31711-3-ville.syrjala@linux.intel.com


Reviewed-by: Jani Nikula <jani.nikula@intel.com>