Tgsi nir fixes

Created by: PabloPL

Those were found during my work on getting scissor working.

First patch enables shader conversion from tgsi to nir (based on vc4/freedreno). It can be reproduced by starting piglit test

$ PIGLIT_PLATFORM=gbm ./bin/gl-1.0-scissor-copypixels -auto -fbo

Last patch enables lowering ffma. It fixes following error

$ PIGLIT_PLATFORM=gbm ./bin/gl-1.0-scissor-offscreen -auto -fbo
gpir: unsupport nir_op 66