drm/i915: Clean up DVO pipe select bits

Parametrize the DVO pipe select bits.

For consistency with the new way of doing things, let's read out the
pipe select bits even when the port is disable, even though we don't
need that behaviour for asserts in this case.

v2: Order the defines shift,mask,value (Jani)

Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180514172423.9302-5-ville.syrjala@linux.intel.com