intel: Add support for userptr objects

Allow userptr objects to be created and used via libdrm_intel.

At the moment tiling and mapping to GTT aperture is not supported
due hardware limitations across different generations and uncertainty
about its usefulness.

v2: Improved error handling in feature detection per review comments.

v3: Rebase on top of the drm_public addition, minor whitespace addition.

Reviewed-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Damien Lespiau <damien.lespiau@intel.com> (v3)
Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com> (v1,v2)