drm/i915: Clean up DP pipe select bits

Clean up the DP pipe select bits. To make the whole situation a bit
less ugly we'll start to share the same code between .get_hw_state(),
the port state asserts, and the VLV power sequencer code.

v2: Return PIPE_A for cpt/ppt when the port isn't selected by
    any transcoder. Returning INVALID_PIPE explodes *somewhere*
    on some machines (can't immediately see where though). This
    now matches the old behaviour.
v3: Order the defines shift,mask,value (Jani)

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180518152931.13104-4-ville.syrjala@linux.intel.com
Reviewed-by: Jani Nikula <jani.nikula@intel.com>