dri/nouveau: don't use nested functions

dri/nouveau: don't use nested functions

It's a GNU extension that isn't supported by clang right now:
http://gcc.gnu.org/onlinedocs/gcc-4.6.3/gcc/Nested-Functions.html
http://clang.llvm.org/docs/UsersManual.html#c_unimpl_gcc

With this, clang now compiles the nouveau classic driver.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=44061

(Types changed from e.g. 'unsigned char' to 'GLubyte' so that the types can
be concatenated to form a unique function name without any whitespace
interfering.)

[ Francisco Jerez: give meaningful names to the dispatch functions. ]