headergen2: add back __MASK/__SHIFT

fdre needs the __MASK values in a few places..  Since we add these back
in, I should probably just add a cmdline option to original headergen to
dump out the static-inline fxns (and use enums instead of #defines for
<enum>).  That would be less invasive, and probably actually work
properly for nested bitsets.
a2xx: few misc missing parts

Everything that was missing to compile fdre without freedreno_a2xx_reg.h