panfrost/decode: Decode blend constant

panfrost/decode: Decode blend constant

This adds a forgotten decode line on Midgard and adds the field of a
blend constant on Bifrost. The Bifrost encoding is fairly weird; whereas
Midgard is just a regular 32-bit float, Bifrost uses a fancy
fixed-point-esque encoding. The decode logic here is experimentally
correct. The encode logic is a sort of "guesstimate", assuming that the
high byte is just int(f / 255.0) and then solving algebraicly for the
low byte. This might be slightly off in some cases.

Signed-off-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Reviewed-by: Ryan Houdek <Sonicadvance1@gmail.com>