mesa/st: enable ARB_gpu_shader5 if the reported GLSL version >= 400

mesa/st: enable ARB_gpu_shader5 if the reported GLSL version >= 400

The ARB_gpu_shader5 extension is made up of a lot of small sub-parts.
Instead of adding PIPE_CAP's for each of these, just rely on the GLSL
version reported by the pipe driver. The remaining extensions lend
themselves naturally to being checked through a single CAP.

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>