libdrm/intel: execbuf2 support

This patch to libdrm adds support for the new execbuf2 ioctl.  If
detected, it will be used instead of the old ioctl.  By using the new
drm_intel_bufmgr_gem_enable_fenced_relocs(), you can indicate that any
time a fence register is actually required for a relocation target you
will call drm_intel_bo_emit_reloc_fence instead of
drm_intel_bo_emit_reloc, which will reduce fence register pressure.

Signed-off-by: Eric Anholt <eric@anholt.net>