target-helpers: Do not build kms_dri on libdrm-less platforms.

Fix build since 3b176c441b7ddc5f7d2f891da3f76cf3c1814ce1 for
dri_platform=none hosts.

Signed-off-by: Jon TURNEY <jon.turney@dronecode.org.uk>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>
r600g/radeonsi: Reduce or even drop special treatment of persistent mappings

Reviewed-by: Marek Olšák <marek.olsak@amd.com>
r600g/radeonsi: Prefer VRAM for CPU -> GPU streaming buffers

Reviewed-by: Marek Olšák <marek.olsak@amd.com>