mesa: Replace _mesa_rgba_logicop_enabled(ctx) with ctx->Color.ColorLogicOpEnabled

mesa: Replace _mesa_rgba_logicop_enabled(ctx) with ctx->Color.ColorLogicOpEnabled

Since GL_EXT_blend_logic_op is removed, _mesa_rgba_logicop_enabled(ctx)
just returns ctx->Color.ColorLogicOpEnabled.  That seems kind of silly.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Roland Scheidegger <sroland@vmware.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Marek Olšák <maraeo@gmail.com>