st/mesa: remove the "Gallium 0.4 on" prefix from GL_RENDERER

st/mesa: remove the "Gallium 0.4 on" prefix from GL_RENDERER

If you want to keep it for your driver, please raise your hand.
The prefix will probably have to be added into the driver instead of here.

I cringe when I look at my long renderer string:
  Gallium 0.4 on AMD Radeon R9 Fury Series (DRM 3.17.0 / 4.11.0-staging-01277-gab25a9e, LLVM 5.0.0)

I'm sincerely sorry for all apps that detect Mesa by expecting "Gallium"
in the string.

Reviewed-by: Eric Anholt <eric@anholt.net>