drm/i915/fdi: use -EAGAIN instead of local special return value

Using standard -EAGAIN should be perfectly fine instead of using a
special case value.

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/20210930093229.28598-1-jani.nikula@intel.com