drm/i915: Unduplicate CHV phy-releated pre pll enabling code

The same logic is used for DP and HDMI so move it to intel_dpio_phy.c.

v2: Rebase

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/1461761065-21195-5-git-send-email-ander.conselvan.de.oliveira@intel.com