drm/i915: Unduplicate CHV pre-encoder enabling phy logic

The only difference between the DP and HDMI versions was the lane count.
Since lane_count is now set appropriately for HDMI too, get rid of the
duplication and move this to intel_dpio_phy.c

v2: Don't move comments about 2nd common lane staying alive. (Ville)
Signed-off-by: Ander Conselvan de Oliveira <ander.conselvan.de.oliveira@intel.com>
Reviewed-by: Jim Bride <jim.bride@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1461761065-21195-6-git-send-email-ander.conselvan.de.oliveira@intel.com