drm/i915/gtt: Remove _single from page table allocator

We are always allocating a single page. No need to be verbose so
remove the suffix.

Signed-off-by: Mika Kuoppala <mika.kuoppala@intel.com>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Reviewed-by: Michel Thierry <michel.thierry@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>