drm/i915: Restore missing command flush before interrupt on BLT ring

We always skipped flushing the BLT ring if the request flush did not
include the RENDER domain. However, this neglects that we try to flush
the COMMAND domain after every batch and before the breadcrumb interrupt
(to make sure the batch is indeed completed prior to the interrupt
firing and so insuring CPU coherency). As a result of the missing flush,
incoherency did indeed creep in, most notable when using lots of command
buffers and so potentially rewritting an active command buffer (i.e.
the GPU was still executing from it even though the following interrupt
had already fired and the request/buffer retired).

As all ring->flush routines now have the same preconditions, de-duplicate
and move those checks up into i915_gem_flush_ring().

Fixes gem_linear_blit.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=35284


Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Tested-by:  <mengmeng.meng@intel.com>