i965: Add support for all 8 possible ARB_draw_buffers in Mesa.

We should be able to do 16, but are limited by Mesa's static buffer
allocations.

src/mesa/drivers/dri/i965/brw_context.h

@@ -252,7 +252,7 @@ struct brw_vs_ouput_sizes {
 #define BRW_MAX_TEX_UNIT 16
 
 /** Max number of render targets in a shader */
-#define BRW_MAX_DRAW_BUFFERS 4
+#define BRW_MAX_DRAW_BUFFERS 8
 
 /**
  * Size of our surface binding table for the WM.

src/mesa/drivers/dri/intel/intel_context.c

@@ -716,7 +716,6 @@ intelInitContext(struct intel_context *intel,
    _mesa_init_point(ctx);
 
    meta_init_metaops(ctx, &intel->meta);
-   ctx->Const.MaxColorAttachments = 4;  /* XXX FBO: review this */
    if (intel->gen >= 4) {
       if (MAX_WIDTH > 8192)
 	 ctx->Const.MaxRenderbufferSize = 8192;