i965: Enable GL_EXT_gpu_shader4 on Gen6+

It's already enabled for all gallium drivers that support GLSL 1.40 or
above and we already support everything in our compiler on SNB+

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>

src/mesa/drivers/dri/i965/intel_extensions.c

       ctx->Extensions.ARB_texture_gather = true;
       ctx->Extensions.ARB_texture_multisample = true;
       ctx->Extensions.ARB_uniform_buffer_object = true;
+      ctx->Extensions.EXT_gpu_shader4 = true;
       ctx->Extensions.EXT_texture_shadow_lod = true;
 
       if (ctx->API != API_OPENGL_COMPAT ||