drm/i915/gem: Take local vma references for the parser

Take and hold a reference to each of the vma (and their objects) as we
process them with the cmdparser. This stops them being freed during the
work if the GEM execbuf is interrupted and the request we expected to
keep the objects alive is incomplete.

Fixes: 686c7c35

 ("drm/i915/gem: Asynchronous cmdparser")
Closes: https://gitlab.freedesktop.org/drm/intel/issues/970
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200113154555.1909639-1-chris@chris-wilson.co.uk