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