drm/i915: Disconnect 32 and 48 bit ppGTT support

Configurations like virtualized environments may support only 48 bit
ppGTT without supporting 32 bit ppGTT. Support this by disconnecting
the relationship of the two feature bits.

Cc: Tina Zhang <tina.zhang@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Zhi Wang <zhi.a.wang@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>