drm/i915: Enforce execobject.alignment to be a power-of-two

Internal requirement for the alignment is that it must be a
power-of-two, so enforce rejection at the user interface to execbuffer
(which allows the caller to specify a stricter-than-expected alignment
criterion).
Signed-off-by: NChris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: NDamien Lespiau <damien.lespiau@intel.com>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_gem_execbuffer.c

@@ -957,6 +957,9 @@ validate_exec_list(struct drm_device *dev,
 		if (exec[i].flags & invalid_flags)
 			return -EINVAL;
 
+		if (exec[i].alignment && !is_power_of_2(exec[i].alignment))
+			return -EINVAL;
+
 		/* First check for malicious input causing overflow in
 		 * the worst case where we need to allocate the entire
 		 * relocation tree as a single array.