About 10 years ago, the code to control dithering in the Linux driver for Intel graphics was contained in the source code for these two files, drivers/gpu/drm/i915/i915_reg.h and drivers/gpu/drm/i915/intel_display.c
I know today they are using different drivers, however maybe the hardware registers haven't changed since that time.
Could someone with Linux using Intel graphics hardware on a laptop with a 6-bit panel please load up a gradient image and try these commands to see if the image changes?
% sudo intel_reg_write 0x70008 0xc0000040 # no dither
% sudo intel_reg_write 0x70008 0xc0000050 # spatial
% sudo intel_reg_write 0x70008 0xc0000054 # ST
Edit: Before doing that, could you please also make a note of what the current register setting is before you change values? intel_reg_read 0x70008