drm/i915: add vlv_pipe_to_phy() helper to replace DPIO_PHY()

Add a helper with better typing and handing for bogus input, and better
in line with vlv_dig_port_to_channel(), vlv_dig_port_to_phy(), and
vlv_pipe_to_channel().

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231114104534.4180144-2-jani.nikula@intel.com