dri: Fix i965 build

dri: Fix i965 build

The following commit broke the i965 build:

    commit 4a486f8bf2ca3d88228f8313282289abe78bc2f8
    Author: Marek Olšák <maraeo@gmail.com>
    Date:   Fri Nov 23 18:31:42 2012 +0100

    glx/dri2: add and use new driver hook flush_with_flags

That commit added a forward declaration of enum __DRI2throttleReason to
dri_interface.h. C++ 98 does not allow forward declarations of enums.

The fix: Move the enum's definition to earlier in the file.

Signed-off-by: Chad Versace <chad.versace@linux.intel.com>