st/dri: unwrap/remove __NOT_HAVE_DRM_H magic

st/dri: unwrap/remove __NOT_HAVE_DRM_H magic

With the dri_interface.h clean of the macro, we can remove the final
only st/dri specific use of the very same.

Seemingly it was incorrectly used, as the build-time presence of dri2 is
not libdrm specific. At run-time, the code is already limited to dri2
use-cases plus returning true, when the extension is not present (or too
old) will likely lead to a crash as one tries to use it shortly after
the dri_with_format() call.

As a side effect this gives us a nice cleanup the builds.

Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>