gallium/radeon: add the kernel version into the renderer string

gallium/radeon: add the kernel version into the renderer string

Example:
Gallium 0.4 on AMD TONGA (DRM 3.2.0 / 4.5.0, LLVM 3.9.0)

My kernel version is pretty long already (4.5.0-amd-01025-g32791c1)
and adding "kernel" into the string would make too it long for glxinfo
to display.

Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>