drm/i915/gtt: Return struct i915_scratch_page from alloc_scratch

Every other alloc_* function return the pointer to the page
they alloc. Follow the convention with scratch page also.

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>

diff --cc drivers/gpu/drm/i915/i915_gem_gtt.c
Simple merge