v3d: Explicitly expose OpenGL ES Shading Language 3.1

This will expose GL_EXT_primitive_bounding_box and
GL_OES_primitive_bounding_box after previous commits
expose OpenGL ES 3.1 once Compute Shaders are available.

Reviewed-by: Eric Anholt <eric@anholt.net>
49 jobs for !2368 with v3d-gles31-enable in 22 minutes and 19 seconds (queued for 6 seconds)
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