drm/i915: split gen8+ flush and bb_start emission functions

drm/i915: split gen8+ flush and bb_start emission functions

These functions are independent from the backend used and can therefore
be split out of the exelists submission file, so they can be re-used by
the upcoming GuC submission backend.

Based on a patch by Chris Wilson.

Signed-off-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
Reviewed-by: John Harrison <John.C.Harrison@Intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201209233618.4287-3-chris@chris-wilson.co.uk
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>