panfrost/midgard: Add fround(_even), ftrunc, ffma

panfrost/midgard: Add fround(_even), ftrunc, ffma

These ops were discovered by invoking the correspondingly names GLSL
functions. The rounding ops here behave exact as expected and are mapped
to their corresponding NIR ops where applicable. The ffma behaves as a
LUT instruction and requires some special argument packing (since
Midgard normally only allows for 2 arguments); this quirk will be
addressed in the future, but for now FMA is still lowered.

Signed-off-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>