drm/i915: Update i915 uapi documentation

Add some missing i915 uapi documentation which the new
i915 VM_BIND feature documentation will be refer to.

Signed-off-by: Niranjana Vishwanathapura <niranjana.vishwanathapura@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220701003110.24843-3-niranjana.vishwanathapura@intel.com