DocBook/drm: Streamline wording of GEM initialization

	  GEM is an alternative to TTM, designed specifically for UMA

	  devices.  It has simpler initialization and execution requirements

	  than TTM, but has no VRAM management capability.  Core GEM

	  initialization is comprised of a basic drm_mm_init call to create

	  is initialized by calling drm_mm_init() to create

	  a GTT DRM MM object, which provides an address space pool for

	  object allocation.  In a KMS configuration, the driver

	  needs to allocate and initialize a command ring buffer following