drm/i915/fdi: move fdi bc bifurcation functions to intel_fdi.c

Move FDI related functions to intel_fdi.c. Don't bother with renaming as
we'll make the functions static shortly.

Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/fd8afe4876f0b0762a9c69e01762a8dba31349e5.1629906431.git.jani.nikula@intel.com