i965: Use current draw buffer instead of drawable visual to get alpha bits

Use the currently bound draw buffer instead of the visual from the
drawable used to create the context.  This cause problems generating
mipmaps for an RGBA texture in an RGB context.

This fixes the failure in piglit's glsl-lod-bias test reported in bug #25614.

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

@@ -158,9 +158,9 @@ cc_unit_populate_key(struct brw_context *brw, struct brw_cc_unit_key *key)
 
       /* If the renderbuffer is XRGB, we have to frob the blend function to
        * force the destination alpha to 1.0.  This means replacing GL_DST_ALPHA
-       * with GL_ONE and GL_ONE_MINUS_DST_ALPAH with GL_ZERO.
+       * with GL_ONE and GL_ONE_MINUS_DST_ALPHA with GL_ZERO.
        */
-      if (ctx->Visual.alphaBits == 0) {
+      if (ctx->DrawBuffer->Visual.alphaBits == 0) {
 	 key->blend_src_rgb = fix_xRGB_alpha(key->blend_src_rgb);
 	 key->blend_src_a   = fix_xRGB_alpha(key->blend_src_a);
 	 key->blend_dst_rgb = fix_xRGB_alpha(key->blend_dst_rgb);

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

@@ -581,7 +581,7 @@ brw_update_renderbuffer_surface(struct brw_context *brw,
    /* As mentioned above, disable writes to the alpha component when the
     * renderbuffer is XRGB.
     */
-   if (ctx->Visual.alphaBits == 0)
+   if (ctx->DrawBuffer->Visual.alphaBits == 0)
      key.color_mask[3] = GL_FALSE;
 
    key.color_blend = (!ctx->Color._LogicOpEnabled &&