drm/i915: fix simple_return.cocci warnings

drivers/gpu/drm/i915/i915_gem_gtt.c:1349:1-4: WARNING: end returns can be simpified and declaration on line 1347 can be dropped

 Simplify a trivial if-return sequence.  Possibly combine with a
 preceding function call.
Generated by: scripts/coccinelle/misc/simple_return.cocci

CC: Michel Thierry <michel.thierry@intel.com>
Signed-off-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>