drm/i915: Move DPIO phy documentation section to intel_dpio_phy.c

Move the DPIO phy documentation section to intel_dpio_phy.c, since that
is a more suitable place now that there is a source file dedicated for
those phys.

Signed-off-by: Ander Conselvan de Oliveira <ander.conselvan.de.oliveira@intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/55a2d38c15c06a8c5bce498b28decc03948f0224.1475770848.git-series.ander.conselvan.de.oliveira@intel.com