dri/common: remove unused drm_version variable

dri/common: remove unused drm_version variable

As of last commit the only user of it (radeon/r200) no longer uses it.
As such let's remove it and cleanup the nasty hacks that we had in place
to support this.

v2: Leave LIBDRM_CFLAGS around.

Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Reviewed-by: Michel Dänzer <michel.daenzer@amd.com> (v1)
Reviewed-by: Marek Olšák <marek.olsak@amd.com> (v1)