i965/batch: avoid reverting batch buffer if saved state is an empty

i965/batch: avoid reverting batch buffer if saved state is an empty

There's no point reverting to the last saved point if that save point is
the empty batch, we will just repeat ourselves.

v2: Merge with new commits, changes was minimized, added the 'fixes' tag
v3: Added in to patch series
v4: Fixed the regression which was introduced by this patch
    Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=108630
    Reported-by:  Mark Janes <mark.a.janes@intel.com>
    The solution provided by: Jordan Justen <jordan.l.justen@intel.com>

CC: Chris Wilson <chris@chris-wilson.co.uk>
Fixes: 3faf56ffbdeb "intel: Add an interface for saving/restoring
                     the batchbuffer state."
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=107626
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=108630 (fixed in v4)
Signed-off-by: Andrii Simiklit <andrii.simiklit@globallogic.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>