drm/i915/fdi: move intel_update_fdi_pll_freq to intel_fdi.c

Move FDI related functions to intel_fdi.c. Rename to have intel_fdi
prefix while at it.

Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/da1609dfce4623f8ec86254aea6c2c8679b6a37f.1629906431.git.jani.nikula@intel.com