drm/i915/fdi: move fdi modeset asserts to intel_fdi.c

Keep the functionality and the assert code together.

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/427d27eb4e5daca208d496d6c2ffc91ed90ba714.1632992608.git.jani.nikula@intel.com