drm/i915: Clean up TV pipe select bits

Parametrize the TV 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)
    Clear the stale pipe select bit in load detection (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/20180514172423.9302-4-ville.syrjala@linux.intel.com