plug-ins: fix not converting 8 bps grayscale MINISWHITE TIFF

We only set tiff_mode for images with bps < 8, but we also use it for
8 bps grayscale images to detect if MINISWHITE needs to be converted.

So, let's always set tiff_mode for PALETTE, MINISBLACK and MINISWHITE.

(cherry picked from commit 2e4b9bcb)