Fix 965+ rendering issues with DRI disabled.
The new chips no longer automatically flush the rendering cache, so if we don't flush the RC at blockhandler, the last rendering done may not appear on the screen. This was particularly noticable with a bare Xorg with some missing root weave, and terminals where the last character wouldn't appear until the cursor blinked. A flush in the DRI blockhandler path had hidden this issue for most people.
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