drm/i915: Refactor translate_signal_level()

drm/i915: Refactor translate_signal_level()

Convert the big switch statement in translate_signal_level() into a neat
table. The table also serves as documentation for the translation
tables. We'll also have other uses for this table later on.

v2: Remove superfluous space (David)

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170223173507.17600-2-ville.syrjala@linux.intel.com
Reviewed-by: David Weinehall <david.weinehall@linux.intel.com>