drm/i915/gtt: Correct unshifted 'from' for gen8_ppgtt_alloc errors

Since the underlying __gen8_ppgtt_clear takes the shifted address, we
must remember to provide it with the shifted original start address.
Reported-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Abdiel Janulgue <abdiel.janulgue@linux.intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Tested-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Reviewed-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190719131524.827-1-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/i915_gem_gtt.c

@@ -1103,7 +1103,7 @@ static int __gen8_ppgtt_alloc(struct i915_address_space * const vm,
 static int gen8_ppgtt_alloc(struct i915_address_space *vm,
 			    u64 start, u64 length)
 {
-	u64 from = start;
+	u64 from;
 	int err;
 
 	GEM_BUG_ON(!IS_ALIGNED(start, BIT_ULL(GEN8_PTE_SHIFT)));
@@ -1112,6 +1112,7 @@ static int gen8_ppgtt_alloc(struct i915_address_space *vm,
 	start >>= GEN8_PTE_SHIFT;
 	length >>= GEN8_PTE_SHIFT;
 	GEM_BUG_ON(length == 0);
+	from = start;
 
 	err = __gen8_ppgtt_alloc(vm, i915_vm_to_ppgtt(vm)->pd,
 				 &start, start + length, vm->top);