drm/i915/gtt: Reorder page alloc/free/init functions

Maintain base page handling functions in order of
alloc, free, init. No functional changes.

v2: s/Introduce/Maintain (Michel)
v3: Rebase

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