drm/i915/gtt: Return struct i915_scratch_page from alloc_scratch
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 <cd221c71e765d1ce593d15b204fee351fb09fd98@intel.com>
Reviewed-by: Michel Thierry <c65ff7d3dfaea3345301797f027f123ededb124c@intel.com>
Signed-off-by: Daniel Vetter <c1b6782c4af8f0673da8923a0702a1832e5940f4@ffwll.ch>

KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs