dri/radeon: use a const __DRIextension array

dri/radeon: use a const __DRIextension array

Rather than keeping a separate and unused copy of the screen extensions
within the radeon screen, use a constant array that can be used directly
with __DRIscreen.

[Kristian Høgsberg]
The copy in the radeon screen isn't unused, that's where the array is
built and stored, the dri screen just points to that.  The pattern
here was used for cases where the extensions exported by a dri driver
could vary at runtime, for example depending on chipset.  In this
case, it's known at compile time, so it makes sense to use a static
const array instead.

Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Kristian Høgsberg <krh@bitplanet.net>