drm/i915/vma: fix kernel-doc function name for i915_vma_size()

drivers/gpu/drm/i915/i915_vma.h:145: warning: expecting prototype for i915_vma_offset(). Prototype was for i915_vma_size() instead

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/9fd58398ba6f86c55cc7a7c62efeab70c3311d59.1683041799.git.jani.nikula@intel.com