drm/i915: Perform execbuffer object locking as a separate step

To help avoid evicting already resident buffers from the batch we're
processing, perform locking as a separate step.

Signed-off-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Reviewed-by: Ramalingam C <ramalingam.c@intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210615113600.30660-1-thomas.hellstrom@linux.intel.com