util, modetest: add support for low-color frame buffer formats

A long outstanding issue with the DRM subsystem has been the lack of support for low-color displays, as used typically on older desktop systems, and on small embedded displays.

This merge request adds support for color-indexed frame buffer formats with 2, 4, and 16 colors. It has been tested on ARAnyM using a work-in-progress Atari DRM driver.

https://lore.kernel.org/r/cover.1690537375.git.geert@linux-m68k.org