postgres provider: fix b85af12c
Reactivated 8bit and 1bit mode for QGIS Server

The commit 76d1f955c019c3d42cbab29ce2d147acd233b54a simplified the
mFormat private attribute to PNG, JPG, PDF, etc and created
mFormatString attribute. The second is used for base64 but not for mode.
This commit corrected it.