drm/i915: Make adjusted_mode.clock non-pixel multiplied

It would be easier if adjusted_mode.clock would be the pipe pixel clock,
and it actually is, except for the cases where pixel_multiplier > 1.

So let's change intel_sdvo to use port_clock as the multiplied clock,
and then we can leave adjusted_mode.clock as pipe pixel clock.

v2: Improve port_clock documentation
    Rebased on top of SDVO pixel_multiplier fixes

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>