drm/i915: Clean up LVDS pipe select bits

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

v2: Order the defines shift,mask,value (Jani)
    Drop ruperfluous braces and whitesapce changes (Jani)
    Combine masks in compute_is_dual_link_lvds() (Jani)
v3: Fix LVDS_PIPE_SEL_MASK_CPT

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/20180514182827.28629-1-ville.syrjala@linux.intel.com

drivers/gpu/drm/i915/intel_drv.h

@@ -1824,6 +1824,8 @@ void intel_infoframe_init(struct intel_digital_port *intel_dig_port);
 
 
 /* intel_lvds.c */
+bool intel_lvds_port_enabled(struct drm_i915_private *dev_priv,
+			     i915_reg_t lvds_reg, enum pipe *pipe);
 void intel_lvds_init(struct drm_i915_private *dev_priv);
 struct intel_encoder *intel_get_lvds_encoder(struct drm_device *dev);
 bool intel_is_dual_link_lvds(struct drm_device *dev);