drm/i915: Unduplicate CHV signal level code

The code for programming voltage swing and emphasis was duplicated
between DP and HDMI code. Move that to a new file, intel_dpio_phy.c.

v2: Keep the "Use 800mV-0dB" comment in the HDMI code. (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/1461761065-21195-3-git-send-email-ander.conselvan.de.oliveira@intel.com