In b9c095cc ("panfrost: Rewrite the clear colour packing code"),
packing of clear colours was corrected to use the tilebuffer's
fractional bits, fixing dithering of the clear colour with formats like
RGB565. Unfortunately, that commit did so unconditionally. If the
framebuffer is dithered, but dithering is disabled at the time of
the clear, we would incorrectly dither the clear.

This is a regression, as the old (broken) code passed the relevant CTS
test. What's the catch? Depending on dither state, there are two
formulas to pack tilebuffer colours. We need to handle both. Fixes
KHR-GLES31.core.draw_buffers_indexed.color_masks.

Fixes: b9c095cc

 ("panfrost: Rewrite the clear colour packing code")
Signed-off-by: Alyssa Rosenzweig <alyssa@collabora.com>