drm/i915: use new iterator in i915_gem_object_wait_reservation

Simplifying the code a bit.

Signed-off-by: Christian König <christian.koenig@amd.com>
[mlankhorst: Handle timeout = 0 correctly, use new i915_request_wait_timeout.]
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Acked-by: Daniel Vetter <daniel@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20211116102431.198905-7-christian.koenig@amd.com