mesa: fix GLES 3.1 version calculation

mesa: fix GLES 3.1 version calculation

All of ARB_gpu_shader5 is most certainly not required for GLES 3.1
(most of it is in OES_gpu_shader5 on top of GLES 3.1).

Some of what is required from ARB_gpu_shader5 is provided by
ARB_texture_gather, so check for that.  The remaining subset of
ARB_gpu_shader5 doesn't have individual extensions to check for,
but I guess it is unlikely that some driver has all of these
extensions but not, say, integer bitfield manipulation.

Signed-off-by: Rob Clark <robdclark@gmail.com>
Acked-by: Ilia Mirkin <imirkin@alum.mit.edu>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>