i965: Enable ARB_derivative_control on Gen7+.

The extension says GL 4.0 is required. We'll meet the spirit
of that restriction by enabling on just those generations which will
soon support GL 4.0 (Gen7+), although it's technically supportable on
all generations.

Signed-off-by: Chris Forbes <chrisf@ijw.co.nz>
Reviewed-by: Matt Turner <mattst88@gmail.com>

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

@@ -305,6 +305,7 @@ intelInitExtensions(struct gl_context *ctx)
       }
 
       ctx->Extensions.ARB_texture_compression_bptc = true;
+      ctx->Extensions.ARB_derivative_control = true;
    }
 
    if (brw->gen >= 8) {