configure/freedreno: make KGSL support optional

libdrm_freedreno currently supports two backends, 'msm' for the upstream
drm/msm driver, and 'kgsl' which supports (to some extent), the android/
downstream kgsl driver plus a sort of drm shim nonsense to get flink
names.

However, kgsl support is strictly on a best-effort basis.  Different
android devices with different versions of kgsl may have different
abi's.  And the existing kgsl interface (at least the parts of it that
we use) is completely broken for 64bit.  Lets disable it by default lest
anyone actually try to use it.

Signed-off-by: Rob Clark <robclark@freedesktop.org>