drm/i915: Create a struct to hold information about the broxton phys

Information about which phy is dual channel is hardcoded in the phy init
sequence. Split that to a separate struct so the init sequence is more
generic.

v2: Restore mangled part that ended up in following patch. (Imre)

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/9102f4c984044126057e4fdd1b91a615ff25fae6.1475770848.git-series.ander.conselvan.de.oliveira@intel.com