drm/i915/uapi: introduce drm_i915_gem_create_ext

Same old gem_create but with now with extensions support. This is needed
to support various upcoming usecases.

v2:(Chris)
    - Use separate ioctl number for gem_create_ext, instead of hijacking
      the existing gem_create ioctl, otherwise we run into the issue
      with being unable to detect if the kernel supports the new extension
      behaviour.
    - We now have gem_create_ext.flags, which should be zeroed.
    - I915_GEM_CREATE_EXT_SETPARAM value is now zero, since this is the
      index into our array of extensions.
    - Setup a "vanilla" object which we can directly apply our extensions
      to.
v3:(Daniel & Jason)
    - drop I915_GEM_CREATE_EXT_SETPARAM. Instead just have each extension
      do one thing only, instead of generic setparam which can cover
      various use cases.
    - add some kernel-doc.

Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Signed-off-by: CQ Tang <cq.tang@intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Cc: Lionel Landwerlin <lionel.g.landwerlin@linux.intel.com>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Daniel Vetter <daniel.vetter@intel.com>
Cc: Kenneth Graunke <kenneth@whitecape.org>
Cc: Jason Ekstrand <jason@jlekstrand.net>
Cc: Dave Airlie <airlied@gmail.com>
Cc: dri-devel@lists.freedesktop.org
Cc: mesa-dev@lists.freedesktop.org
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20210429103056.407067-5-matthew.auld@intel.com