drm/i915: Remove redundant list_empty(eb->vmas) tests in execbuffer

Part of the pre-validation for an execbuffer call is that there is at
least one object in the execlist. As we bail if we fail to lookup any
object, we can be sure that after the eb_lookup_vma() there is at least
one object in the vma list and so we do not need to assert.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Ben Widawsky <benjamin.widawsky@intel.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>