tdf#76941 : image greyscale not imported correctly

The eColorMode and other graphic attributes were never checked due to this statement.
This statement is last altered by 0f0a22ad
This only fixes the import.

Change-Id: I9ba7e745582faf37898f284600d638aa4806a362
Reviewed-on: https://gerrit.libreoffice.org/15710Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>