drm/i915: Introduce intel_ddi_dp_level.

No functional changes. This only moves the DP level
selection to a separated function that will be later
used to organize better the vswing sequences.

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170829232230.23051-1-rodrigo.vivi@intel.com